diff -r 48c4eec2b7e6 -r 8c2e4d02f4ef wp/wp-includes/html-api/class-wp-html-processor.php
--- a/wp/wp-includes/html-api/class-wp-html-processor.php	Fri Sep 05 18:40:08 2025 +0200
+++ b/wp/wp-includes/html-api/class-wp-html-processor.php	Fri Sep 05 18:52:52 2025 +0200
@@ -97,22 +97,11 @@
  * will abort early and stop all processing. This draconian measure ensures
  * that the HTML Processor won't break any HTML it doesn't fully understand.
  *
- * The following list specifies the HTML tags that _are_ supported:
+ * The HTML Processor supports all elements other than a specific set:
  *
- *  - Containers: ADDRESS, BLOCKQUOTE, DETAILS, DIALOG, DIV, FOOTER, HEADER, MAIN, MENU, SPAN, SUMMARY.
- *  - Custom elements: All custom elements are supported. :)
- *  - Form elements: BUTTON, DATALIST, FIELDSET, INPUT, LABEL, LEGEND, METER, PROGRESS, SEARCH.
- *  - Formatting elements: B, BIG, CODE, EM, FONT, I, PRE, SMALL, STRIKE, STRONG, TT, U, WBR.
- *  - Heading elements: H1, H2, H3, H4, H5, H6, HGROUP.
- *  - Links: A.
- *  - Lists: DD, DL, DT, LI, OL, UL.
- *  - Media elements: AUDIO, CANVAS, EMBED, FIGCAPTION, FIGURE, IMG, MAP, PICTURE, SOURCE, TRACK, VIDEO.
- *  - Paragraph: BR, P.
- *  - Phrasing elements: ABBR, AREA, BDI, BDO, CITE, DATA, DEL, DFN, INS, MARK, OUTPUT, Q, SAMP, SUB, SUP, TIME, VAR.
- *  - Sectioning elements: ARTICLE, ASIDE, HR, NAV, SECTION.
- *  - Templating elements: SLOT.
- *  - Text decoration: RUBY.
- *  - Deprecated elements: ACRONYM, BLINK, CENTER, DIR, ISINDEX, KEYGEN, LISTING, MULTICOL, NEXTID, PARAM, SPACER.
+ *  - Any element inside a TABLE.
+ *  - Any element inside foreign content, including SVG and MATH.
+ *  - Any element outside the IN BODY insertion mode, e.g. doctype declarations, meta, links.
  *
  * ### Supported markup
  *
@@ -121,15 +110,30 @@
  * may in fact belong _before_ the table in the DOM. If the HTML Processor encounters
  * such a case it will stop processing.
  *
- * The following list specifies HTML markup that _is_ supported:
+ * The following list illustrates some common examples of unexpected HTML inputs that
+ * the HTML Processor properly parses and represents:
+ *
+ *  - HTML with optional tags omitted, e.g. `<p>one<p>two`.
+ *  - HTML with unexpected tag closers, e.g. `<p>one </span> more</p>`.
+ *  - Non-void tags with self-closing flag, e.g. `<div/>the DIV is still open.</div>`.
+ *  - Heading elements which close open heading elements of another level, e.g. `<h1>Closed by </h2>`.
+ *  - Elements containing text that looks like other tags but isn't, e.g. `<title>The <img> is plaintext</title>`.
+ *  - SCRIPT and STYLE tags containing text that looks like HTML but isn't, e.g. `<script>document.write('<p>Hi</p>');</script>`.
+ *  - SCRIPT content which has been escaped, e.g. `<script><!-- document.write('<script>console.log("hi")</script>') --></script>`.
  *
- *  - Markup involving only those tags listed above.
- *  - Fully-balanced and non-overlapping tags.
- *  - HTML with unexpected tag closers.
- *  - Some unbalanced or overlapping tags.
- *  - P tags after unclosed P tags.
- *  - BUTTON tags after unclosed BUTTON tags.
- *  - A tags after unclosed A tags that don't involve any active formatting elements.
+ * ### Unsupported Features
+ *
+ * This parser does not report parse errors.
+ *
+ * Normally, when additional HTML or BODY tags are encountered in a document, if there
+ * are any additional attributes on them that aren't found on the previous elements,
+ * the existing HTML and BODY elements adopt those missing attribute values. This
+ * parser does not add those additional attributes.
+ *
+ * In certain situations, elements are moved to a different part of the document in
+ * a process called "adoption" and "fostering." Because the nodes move to a location
+ * in the document that the parser had already processed, this parser does not support
+ * these situations and will bail.
  *
  * @since 6.4.0
  *
@@ -159,7 +163,7 @@
 	 *
 	 * @var WP_HTML_Processor_State
 	 */
-	private $state = null;
+	private $state;
 
 	/**
 	 * Used to create unique bookmark names.
@@ -189,6 +193,17 @@
 	private $last_error = null;
 
 	/**
+	 * Stores context for why the parser bailed on unsupported HTML, if it did.
+	 *
+	 * @see self::get_unsupported_exception
+	 *
+	 * @since 6.7.0
+	 *
+	 * @var WP_HTML_Unsupported_Exception|null
+	 */
+	private $unsupported_exception = null;
+
+	/**
 	 * Releases a bookmark when PHP garbage-collects its wrapping WP_HTML_Token instance.
 	 *
 	 * This function is created inside the class constructor so that it can be passed to
@@ -197,7 +212,7 @@
 	 *
 	 * @since 6.4.0
 	 *
-	 * @var closure
+	 * @var Closure|null
 	 */
 	private $release_internal_bookmark_on_destruct = null;
 
@@ -212,6 +227,15 @@
 	private $element_queue = array();
 
 	/**
+	 * Stores the current breadcrumbs.
+	 *
+	 * @since 6.7.0
+	 *
+	 * @var string[]
+	 */
+	private $breadcrumbs = array();
+
+	/**
 	 * Current stack event, if set, representing a matched token.
 	 *
 	 * Because the parser may internally point to a place further along in a document
@@ -221,32 +245,17 @@
 	 *
 	 * @since 6.6.0
 	 *
-	 * @var ?WP_HTML_Stack_Event
+	 * @var WP_HTML_Stack_Event|null
 	 */
 	private $current_element = null;
 
 	/**
 	 * Context node if created as a fragment parser.
 	 *
-	 * @var ?WP_HTML_Token
+	 * @var WP_HTML_Token|null
 	 */
 	private $context_node = null;
 
-	/**
-	 * Whether the parser has yet processed the context node,
-	 * if created as a fragment parser.
-	 *
-	 * The context node will be initially pushed onto the stack of open elements,
-	 * but when created as a fragment parser, this context element (and the implicit
-	 * HTML document node above it) should not be exposed as a matched token or node.
-	 *
-	 * This boolean indicates whether the processor should skip over the current
-	 * node in its initial search for the first node created from the input HTML.
-	 *
-	 * @var bool
-	 */
-	private $has_seen_context_node = false;
-
 	/*
 	 * Public Interface Functions
 	 */
@@ -288,30 +297,52 @@
 			return null;
 		}
 
-		$processor                        = new static( $html, self::CONSTRUCTOR_UNLOCK_CODE );
-		$processor->state->context_node   = array( 'BODY', array() );
-		$processor->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_BODY;
-
-		// @todo Create "fake" bookmarks for non-existent but implied nodes.
-		$processor->bookmarks['root-node']    = new WP_HTML_Span( 0, 0 );
-		$processor->bookmarks['context-node'] = new WP_HTML_Span( 0, 0 );
-
-		$processor->state->stack_of_open_elements->push(
-			new WP_HTML_Token(
-				'root-node',
-				'HTML',
-				false
-			)
-		);
-
-		$context_node = new WP_HTML_Token(
-			'context-node',
-			$processor->state->context_node[0],
-			false
-		);
-
-		$processor->state->stack_of_open_elements->push( $context_node );
-		$processor->context_node = $context_node;
+		$context_processor = static::create_full_parser( "<!DOCTYPE html>{$context}", $encoding );
+		if ( null === $context_processor ) {
+			return null;
+		}
+
+		while ( $context_processor->next_tag() ) {
+			if ( ! $context_processor->is_virtual() ) {
+				$context_processor->set_bookmark( 'final_node' );
+			}
+		}
+
+		if (
+			! $context_processor->has_bookmark( 'final_node' ) ||
+			! $context_processor->seek( 'final_node' )
+		) {
+			_doing_it_wrong( __METHOD__, __( 'No valid context element was detected.' ), '6.8.0' );
+			return null;
+		}
+
+		return $context_processor->create_fragment_at_current_node( $html );
+	}
+
+	/**
+	 * Creates an HTML processor in the full parsing mode.
+	 *
+	 * It's likely that a fragment parser is more appropriate, unless sending an
+	 * entire HTML document from start to finish. Consider a fragment parser with
+	 * a context node of `<body>`.
+	 *
+	 * UTF-8 is the only allowed encoding. If working with a document that
+	 * isn't UTF-8, first convert the document to UTF-8, then pass in the
+	 * converted HTML.
+	 *
+	 * @param string      $html                    Input HTML document to process.
+	 * @param string|null $known_definite_encoding Optional. If provided, specifies the charset used
+	 *                                             in the input byte stream. Currently must be UTF-8.
+	 * @return static|null The created processor if successful, otherwise null.
+	 */
+	public static function create_full_parser( $html, $known_definite_encoding = 'UTF-8' ) {
+		if ( 'UTF-8' !== $known_definite_encoding ) {
+			return null;
+		}
+
+		$processor                             = new static( $html, self::CONSTRUCTOR_UNLOCK_CODE );
+		$processor->state->encoding            = $known_definite_encoding;
+		$processor->state->encoding_confidence = 'certain';
 
 		return $processor;
 	}
@@ -348,20 +379,30 @@
 		$this->state = new WP_HTML_Processor_State();
 
 		$this->state->stack_of_open_elements->set_push_handler(
-			function ( WP_HTML_Token $token ) {
+			function ( WP_HTML_Token $token ): void {
 				$is_virtual            = ! isset( $this->state->current_token ) || $this->is_tag_closer();
 				$same_node             = isset( $this->state->current_token ) && $token->node_name === $this->state->current_token->node_name;
 				$provenance            = ( ! $same_node || $is_virtual ) ? 'virtual' : 'real';
 				$this->element_queue[] = new WP_HTML_Stack_Event( $token, WP_HTML_Stack_Event::PUSH, $provenance );
+
+				$this->change_parsing_namespace( $token->integration_node_type ? 'html' : $token->namespace );
 			}
 		);
 
 		$this->state->stack_of_open_elements->set_pop_handler(
-			function ( WP_HTML_Token $token ) {
+			function ( WP_HTML_Token $token ): void {
 				$is_virtual            = ! isset( $this->state->current_token ) || ! $this->is_tag_closer();
 				$same_node             = isset( $this->state->current_token ) && $token->node_name === $this->state->current_token->node_name;
 				$provenance            = ( ! $same_node || $is_virtual ) ? 'virtual' : 'real';
 				$this->element_queue[] = new WP_HTML_Stack_Event( $token, WP_HTML_Stack_Event::POP, $provenance );
+
+				$adjusted_current_node = $this->get_adjusted_current_node();
+
+				if ( $adjusted_current_node ) {
+					$this->change_parsing_namespace( $adjusted_current_node->integration_node_type ? 'html' : $adjusted_current_node->namespace );
+				} else {
+					$this->change_parsing_namespace( 'html' );
+				}
 			}
 		);
 
@@ -370,12 +411,191 @@
 		 * a private method into WP_HTML_Token classes without
 		 * exposing it to any public API.
 		 */
-		$this->release_internal_bookmark_on_destruct = function ( $name ) {
+		$this->release_internal_bookmark_on_destruct = function ( string $name ): void {
 			parent::release_bookmark( $name );
 		};
 	}
 
 	/**
+	 * Creates a fragment processor at the current node.
+	 *
+	 * HTML Fragment parsing always happens with a context node. HTML Fragment Processors can be
+	 * instantiated with a `BODY` context node via `WP_HTML_Processor::create_fragment( $html )`.
+	 *
+	 * The context node may impact how a fragment of HTML is parsed. For example, consider the HTML
+	 * fragment `<td />Inside TD?</td>`.
+	 *
+	 * A BODY context node will produce the following tree:
+	 *
+	 *     └─#text Inside TD?
+	 *
+	 * Notice that the `<td>` tags are completely ignored.
+	 *
+	 * Compare that with an SVG context node that produces the following tree:
+	 *
+	 *     ├─svg:td
+	 *     └─#text Inside TD?
+	 *
+	 * Here, a `td` node in the `svg` namespace is created, and its self-closing flag is respected.
+	 * This is a peculiarity of parsing HTML in foreign content like SVG.
+	 *
+	 * Finally, consider the tree produced with a TABLE context node:
+	 *
+	 *     └─TBODY
+	 *       └─TR
+	 *         └─TD
+	 *           └─#text Inside TD?
+	 *
+	 * These examples demonstrate how important the context node may be when processing an HTML
+	 * fragment. Special care must be taken when processing fragments that are expected to appear
+	 * in specific contexts. SVG and TABLE are good examples, but there are others.
+	 *
+	 * @see https://html.spec.whatwg.org/multipage/parsing.html#html-fragment-parsing-algorithm
+	 *
+	 * @since 6.8.0
+	 *
+	 * @param string $html Input HTML fragment to process.
+	 * @return static|null The created processor if successful, otherwise null.
+	 */
+	private function create_fragment_at_current_node( string $html ) {
+		if ( $this->get_token_type() !== '#tag' || $this->is_tag_closer() ) {
+			_doing_it_wrong(
+				__METHOD__,
+				__( 'The context element must be a start tag.' ),
+				'6.8.0'
+			);
+			return null;
+		}
+
+		$tag_name  = $this->current_element->token->node_name;
+		$namespace = $this->current_element->token->namespace;
+
+		if ( 'html' === $namespace && self::is_void( $tag_name ) ) {
+			_doing_it_wrong(
+				__METHOD__,
+				sprintf(
+					// translators: %s: A tag name like INPUT or BR.
+					__( 'The context element cannot be a void element, found "%s".' ),
+					$tag_name
+				),
+				'6.8.0'
+			);
+			return null;
+		}
+
+		/*
+		 * Prevent creating fragments at nodes that require a special tokenizer state.
+		 * This is unsupported by the HTML Processor.
+		 */
+		if (
+			'html' === $namespace &&
+			in_array( $tag_name, array( 'IFRAME', 'NOEMBED', 'NOFRAMES', 'SCRIPT', 'STYLE', 'TEXTAREA', 'TITLE', 'XMP', 'PLAINTEXT' ), true )
+		) {
+			_doing_it_wrong(
+				__METHOD__,
+				sprintf(
+					// translators: %s: A tag name like IFRAME or TEXTAREA.
+					__( 'The context element "%s" is not supported.' ),
+					$tag_name
+				),
+				'6.8.0'
+			);
+			return null;
+		}
+
+		$fragment_processor = new static( $html, self::CONSTRUCTOR_UNLOCK_CODE );
+
+		$fragment_processor->compat_mode = $this->compat_mode;
+
+		// @todo Create "fake" bookmarks for non-existent but implied nodes.
+		$fragment_processor->bookmarks['root-node'] = new WP_HTML_Span( 0, 0 );
+		$root_node                                  = new WP_HTML_Token(
+			'root-node',
+			'HTML',
+			false
+		);
+		$fragment_processor->state->stack_of_open_elements->push( $root_node );
+
+		$fragment_processor->bookmarks['context-node']   = new WP_HTML_Span( 0, 0 );
+		$fragment_processor->context_node                = clone $this->current_element->token;
+		$fragment_processor->context_node->bookmark_name = 'context-node';
+		$fragment_processor->context_node->on_destroy    = null;
+
+		$fragment_processor->breadcrumbs = array( 'HTML', $fragment_processor->context_node->node_name );
+
+		if ( 'TEMPLATE' === $fragment_processor->context_node->node_name ) {
+			$fragment_processor->state->stack_of_template_insertion_modes[] = WP_HTML_Processor_State::INSERTION_MODE_IN_TEMPLATE;
+		}
+
+		$fragment_processor->reset_insertion_mode_appropriately();
+
+		/*
+		 * > Set the parser's form element pointer to the nearest node to the context element that
+		 * > is a form element (going straight up the ancestor chain, and including the element
+		 * > itself, if it is a form element), if any. (If there is no such form element, the
+		 * > form element pointer keeps its initial value, null.)
+		 */
+		foreach ( $this->state->stack_of_open_elements->walk_up() as $element ) {
+			if ( 'FORM' === $element->node_name && 'html' === $element->namespace ) {
+				$fragment_processor->state->form_element                = clone $element;
+				$fragment_processor->state->form_element->bookmark_name = null;
+				$fragment_processor->state->form_element->on_destroy    = null;
+				break;
+			}
+		}
+
+		$fragment_processor->state->encoding_confidence = 'irrelevant';
+
+		/*
+		 * Update the parsing namespace near the end of the process.
+		 * This is important so that any push/pop from the stack of open
+		 * elements does not change the parsing namespace.
+		 */
+		$fragment_processor->change_parsing_namespace(
+			$this->current_element->token->integration_node_type ? 'html' : $namespace
+		);
+
+		return $fragment_processor;
+	}
+
+	/**
+	 * Stops the parser and terminates its execution when encountering unsupported markup.
+	 *
+	 * @throws WP_HTML_Unsupported_Exception Halts execution of the parser.
+	 *
+	 * @since 6.7.0
+	 *
+	 * @param string $message Explains support is missing in order to parse the current node.
+	 */
+	private function bail( string $message ) {
+		$here  = $this->bookmarks[ $this->state->current_token->bookmark_name ];
+		$token = substr( $this->html, $here->start, $here->length );
+
+		$open_elements = array();
+		foreach ( $this->state->stack_of_open_elements->stack as $item ) {
+			$open_elements[] = $item->node_name;
+		}
+
+		$active_formats = array();
+		foreach ( $this->state->active_formatting_elements->walk_down() as $item ) {
+			$active_formats[] = $item->node_name;
+		}
+
+		$this->last_error = self::ERROR_UNSUPPORTED;
+
+		$this->unsupported_exception = new WP_HTML_Unsupported_Exception(
+			$message,
+			$this->state->current_token->node_name,
+			$here->start,
+			$token,
+			$open_elements,
+			$active_formats
+		);
+
+		throw $this->unsupported_exception;
+	}
+
+	/**
 	 * Returns the last error, if any.
 	 *
 	 * Various situations lead to parsing failure but this class will
@@ -398,11 +618,26 @@
 	 *
 	 * @return string|null The last error, if one exists, otherwise null.
 	 */
-	public function get_last_error() {
+	public function get_last_error(): ?string {
 		return $this->last_error;
 	}
 
 	/**
+	 * Returns context for why the parser aborted due to unsupported HTML, if it did.
+	 *
+	 * This is meant for debugging purposes, not for production use.
+	 *
+	 * @since 6.7.0
+	 *
+	 * @see self::$unsupported_exception
+	 *
+	 * @return WP_HTML_Unsupported_Exception|null
+	 */
+	public function get_unsupported_exception() {
+		return $this->unsupported_exception;
+	}
+
+	/**
 	 * Finds the next tag matching the $query.
 	 *
 	 * @todo Support matching the class name and tag name.
@@ -426,7 +661,7 @@
 	 * }
 	 * @return bool Whether a tag was matched.
 	 */
-	public function next_tag( $query = null ) {
+	public function next_tag( $query = null ): bool {
 		$visit_closers = isset( $query['tag_closers'] ) && 'visit' === $query['tag_closers'];
 
 		if ( null === $query ) {
@@ -456,6 +691,10 @@
 			return false;
 		}
 
+		if ( isset( $query['tag_name'] ) ) {
+			$query['tag_name'] = strtoupper( $query['tag_name'] );
+		}
+
 		$needs_class = ( isset( $query['class_name'] ) && is_string( $query['class_name'] ) )
 			? $query['class_name']
 			: null;
@@ -466,6 +705,10 @@
 					continue;
 				}
 
+				if ( isset( $query['tag_name'] ) && $query['tag_name'] !== $this->get_token_name() ) {
+					continue;
+				}
+
 				if ( isset( $needs_class ) && ! $this->has_class( $needs_class ) ) {
 					continue;
 				}
@@ -499,71 +742,98 @@
 	}
 
 	/**
-	 * Ensures internal accounting is maintained for HTML semantic rules while
-	 * the underlying Tag Processor class is seeking to a bookmark.
+	 * Finds the next token in the HTML document.
 	 *
 	 * This doesn't currently have a way to represent non-tags and doesn't process
 	 * semantic rules for text nodes. For access to the raw tokens consider using
 	 * WP_HTML_Tag_Processor instead.
 	 *
 	 * @since 6.5.0 Added for internal support; do not use.
+	 * @since 6.7.2 Refactored so subclasses may extend.
+	 *
+	 * @return bool Whether a token was parsed.
+	 */
+	public function next_token(): bool {
+		return $this->next_visitable_token();
+	}
+
+	/**
+	 * Ensures internal accounting is maintained for HTML semantic rules while
+	 * the underlying Tag Processor class is seeking to a bookmark.
+	 *
+	 * This doesn't currently have a way to represent non-tags and doesn't process
+	 * semantic rules for text nodes. For access to the raw tokens consider using
+	 * WP_HTML_Tag_Processor instead.
+	 *
+	 * Note that this method may call itself recursively. This is why it is not
+	 * implemented as {@see WP_HTML_Processor::next_token()}, which instead calls
+	 * this method similarly to how {@see WP_HTML_Tag_Processor::next_token()}
+	 * calls the {@see WP_HTML_Tag_Processor::base_class_next_token()} method.
+	 *
+	 * @since 6.7.2 Added for internal support.
 	 *
 	 * @access private
 	 *
 	 * @return bool
 	 */
-	public function next_token() {
+	private function next_visitable_token(): bool {
 		$this->current_element = null;
 
 		if ( isset( $this->last_error ) ) {
 			return false;
 		}
 
-		if ( 'done' !== $this->has_seen_context_node && 0 === count( $this->element_queue ) && ! $this->step() ) {
-			while ( 'context-node' !== $this->state->stack_of_open_elements->current_node()->bookmark_name && $this->state->stack_of_open_elements->pop() ) {
+		/*
+		 * Prime the events if there are none.
+		 *
+		 * @todo In some cases, probably related to the adoption agency
+		 *       algorithm, this call to step() doesn't create any new
+		 *       events. Calling it again creates them. Figure out why
+		 *       this is and if it's inherent or if it's a bug. Looping
+		 *       until there are events or until there are no more
+		 *       tokens works in the meantime and isn't obviously wrong.
+		 */
+		if ( empty( $this->element_queue ) && $this->step() ) {
+			return $this->next_visitable_token();
+		}
+
+		// Process the next event on the queue.
+		$this->current_element = array_shift( $this->element_queue );
+		if ( ! isset( $this->current_element ) ) {
+			// There are no tokens left, so close all remaining open elements.
+			while ( $this->state->stack_of_open_elements->pop() ) {
 				continue;
 			}
-			$this->has_seen_context_node = 'done';
-			return $this->next_token();
-		}
-
-		$this->current_element = array_shift( $this->element_queue );
-		while ( isset( $this->context_node ) && ! $this->has_seen_context_node ) {
-			if ( isset( $this->current_element ) ) {
-				if ( $this->context_node === $this->current_element->token && WP_HTML_Stack_Event::PUSH === $this->current_element->operation ) {
-					$this->has_seen_context_node = true;
-					return $this->next_token();
-				}
-			}
-			$this->current_element = array_shift( $this->element_queue );
+
+			return empty( $this->element_queue ) ? false : $this->next_visitable_token();
 		}
 
-		if ( ! isset( $this->current_element ) ) {
-			if ( 'done' === $this->has_seen_context_node ) {
-				return false;
-			} else {
-				return $this->next_token();
-			}
+		$is_pop = WP_HTML_Stack_Event::POP === $this->current_element->operation;
+
+		/*
+		 * The root node only exists in the fragment parser, and closing it
+		 * indicates that the parse is complete. Stop before popping it from
+		 * the breadcrumbs.
+		 */
+		if ( 'root-node' === $this->current_element->token->bookmark_name ) {
+			return $this->next_visitable_token();
 		}
 
-		if ( isset( $this->context_node ) && WP_HTML_Stack_Event::POP === $this->current_element->operation && $this->context_node === $this->current_element->token ) {
-			$this->element_queue   = array();
-			$this->current_element = null;
-			return false;
+		// Adjust the breadcrumbs for this event.
+		if ( $is_pop ) {
+			array_pop( $this->breadcrumbs );
+		} else {
+			$this->breadcrumbs[] = $this->current_element->token->node_name;
 		}
 
 		// Avoid sending close events for elements which don't expect a closing.
-		if (
-			WP_HTML_Stack_Event::POP === $this->current_element->operation &&
-			! static::expects_closer( $this->current_element->token )
-		) {
-			return $this->next_token();
+		if ( $is_pop && ! $this->expects_closer( $this->current_element->token ) ) {
+			return $this->next_visitable_token();
 		}
 
 		return true;
 	}
 
-
 	/**
 	 * Indicates if the current tag token is a tag closer.
 	 *
@@ -580,7 +850,7 @@
 	 *
 	 * @return bool Whether the current tag is a tag closer.
 	 */
-	public function is_tag_closer() {
+	public function is_tag_closer(): bool {
 		return $this->is_virtual()
 			? ( WP_HTML_Stack_Event::POP === $this->current_element->operation && '#tag' === $this->get_token_type() )
 			: parent::is_tag_closer();
@@ -594,7 +864,7 @@
 	 *
 	 * @return bool Whether the current token is virtual.
 	 */
-	private function is_virtual() {
+	private function is_virtual(): bool {
 		return (
 			isset( $this->current_element->provenance ) &&
 			'virtual' === $this->current_element->provenance
@@ -626,7 +896,7 @@
 	 *                              May also contain the wildcard `*` which matches a single element, e.g. `array( 'SECTION', '*' )`.
 	 * @return bool Whether the currently-matched tag is found at the given nested structure.
 	 */
-	public function matches_breadcrumbs( $breadcrumbs ) {
+	public function matches_breadcrumbs( $breadcrumbs ): bool {
 		// Everything matches when there are zero constraints.
 		if ( 0 === count( $breadcrumbs ) ) {
 			return true;
@@ -639,10 +909,11 @@
 			return false;
 		}
 
-		foreach ( $this->state->stack_of_open_elements->walk_up() as $node ) {
+		for ( $i = count( $this->breadcrumbs ) - 1; $i >= 0; $i-- ) {
+			$node  = $this->breadcrumbs[ $i ];
 			$crumb = strtoupper( current( $breadcrumbs ) );
 
-			if ( '*' !== $crumb && $node->node_name !== $crumb ) {
+			if ( '*' !== $crumb && $node !== $crumb ) {
 				return false;
 			}
 
@@ -667,29 +938,32 @@
 	 *
 	 * @since 6.6.0
 	 *
-	 * @todo When adding support for foreign content, ensure that
-	 *       this returns false for self-closing elements in the
-	 *       SVG and MathML namespace.
-	 *
-	 * @param  ?WP_HTML_Token $node Node to examine instead of current node, if provided.
-	 * @return bool Whether to expect a closer for the currently-matched node,
-	 *              or `null` if not matched on any token.
+	 * @param WP_HTML_Token|null $node Optional. Node to examine, if provided.
+	 *                                 Default is to examine current node.
+	 * @return bool|null Whether to expect a closer for the currently-matched node,
+	 *                   or `null` if not matched on any token.
 	 */
-	public function expects_closer( $node = null ) {
+	public function expects_closer( ?WP_HTML_Token $node = null ): ?bool {
 		$token_name = $node->node_name ?? $this->get_token_name();
+
 		if ( ! isset( $token_name ) ) {
 			return null;
 		}
 
+		$token_namespace        = $node->namespace ?? $this->get_namespace();
+		$token_has_self_closing = $node->has_self_closing_flag ?? $this->has_self_closing_flag();
+
 		return ! (
 			// Comments, text nodes, and other atomic tokens.
 			'#' === $token_name[0] ||
 			// Doctype declarations.
 			'html' === $token_name ||
 			// Void elements.
-			self::is_void( $token_name ) ||
+			( 'html' === $token_namespace && self::is_void( $token_name ) ) ||
 			// Special atomic elements.
-			in_array( $token_name, array( 'IFRAME', 'NOEMBED', 'NOFRAMES', 'SCRIPT', 'STYLE', 'TEXTAREA', 'TITLE', 'XMP' ), true )
+			( 'html' === $token_namespace && in_array( $token_name, array( 'IFRAME', 'NOEMBED', 'NOFRAMES', 'SCRIPT', 'STYLE', 'TEXTAREA', 'TITLE', 'XMP' ), true ) ) ||
+			// Self-closing elements in foreign content.
+			( 'html' !== $token_namespace && $token_has_self_closing )
 		);
 	}
 
@@ -706,7 +980,7 @@
 	 * @param string $node_to_process Whether to parse the next node or reprocess the current node.
 	 * @return bool Whether a tag was matched.
 	 */
-	public function step( $node_to_process = self::PROCESS_NEXT_NODE ) {
+	public function step( $node_to_process = self::PROCESS_NEXT_NODE ): bool {
 		// Refuse to proceed if there was a previous error.
 		if ( null !== $this->last_error ) {
 			return false;
@@ -721,20 +995,18 @@
 			 *
 			 * When moving on to the next node, therefore, if the bottom-most element
 			 * on the stack is a void element, it must be closed.
-			 *
-			 * @todo Once self-closing foreign elements and BGSOUND are supported,
-			 *        they must also be implicitly closed here too. BGSOUND is
-			 *        special since it's only self-closing if the self-closing flag
-			 *        is provided in the opening tag, otherwise it expects a tag closer.
 			 */
 			$top_node = $this->state->stack_of_open_elements->current_node();
-			if ( isset( $top_node ) && ! static::expects_closer( $top_node ) ) {
+			if ( isset( $top_node ) && ! $this->expects_closer( $top_node ) ) {
 				$this->state->stack_of_open_elements->pop();
 			}
 		}
 
 		if ( self::PROCESS_NEXT_NODE === $node_to_process ) {
 			parent::next_token();
+			if ( WP_HTML_Tag_Processor::STATE_TEXT_NODE === $this->parser_state ) {
+				parent::subdivide_text_appropriately();
+			}
 		}
 
 		// Finish stepping when there are no more tokens in the document.
@@ -745,21 +1017,116 @@
 			return false;
 		}
 
-		$this->state->current_token = new WP_HTML_Token(
-			$this->bookmark_token(),
-			$this->get_token_name(),
-			$this->has_self_closing_flag(),
-			$this->release_internal_bookmark_on_destruct
+		$adjusted_current_node = $this->get_adjusted_current_node();
+		$is_closer             = $this->is_tag_closer();
+		$is_start_tag          = WP_HTML_Tag_Processor::STATE_MATCHED_TAG === $this->parser_state && ! $is_closer;
+		$token_name            = $this->get_token_name();
+
+		if ( self::REPROCESS_CURRENT_NODE !== $node_to_process ) {
+			$this->state->current_token = new WP_HTML_Token(
+				$this->bookmark_token(),
+				$token_name,
+				$this->has_self_closing_flag(),
+				$this->release_internal_bookmark_on_destruct
+			);
+		}
+
+		$parse_in_current_insertion_mode = (
+			0 === $this->state->stack_of_open_elements->count() ||
+			'html' === $adjusted_current_node->namespace ||
+			(
+				'math' === $adjusted_current_node->integration_node_type &&
+				(
+					( $is_start_tag && ! in_array( $token_name, array( 'MGLYPH', 'MALIGNMARK' ), true ) ) ||
+					'#text' === $token_name
+				)
+			) ||
+			(
+				'math' === $adjusted_current_node->namespace &&
+				'ANNOTATION-XML' === $adjusted_current_node->node_name &&
+				$is_start_tag && 'SVG' === $token_name
+			) ||
+			(
+				'html' === $adjusted_current_node->integration_node_type &&
+				( $is_start_tag || '#text' === $token_name )
+			)
 		);
 
 		try {
+			if ( ! $parse_in_current_insertion_mode ) {
+				return $this->step_in_foreign_content();
+			}
+
 			switch ( $this->state->insertion_mode ) {
+				case WP_HTML_Processor_State::INSERTION_MODE_INITIAL:
+					return $this->step_initial();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_BEFORE_HTML:
+					return $this->step_before_html();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_BEFORE_HEAD:
+					return $this->step_before_head();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_IN_HEAD:
+					return $this->step_in_head();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_IN_HEAD_NOSCRIPT:
+					return $this->step_in_head_noscript();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_AFTER_HEAD:
+					return $this->step_after_head();
+
 				case WP_HTML_Processor_State::INSERTION_MODE_IN_BODY:
 					return $this->step_in_body();
 
+				case WP_HTML_Processor_State::INSERTION_MODE_IN_TABLE:
+					return $this->step_in_table();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_IN_TABLE_TEXT:
+					return $this->step_in_table_text();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_IN_CAPTION:
+					return $this->step_in_caption();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_IN_COLUMN_GROUP:
+					return $this->step_in_column_group();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_IN_TABLE_BODY:
+					return $this->step_in_table_body();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_IN_ROW:
+					return $this->step_in_row();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_IN_CELL:
+					return $this->step_in_cell();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_IN_SELECT:
+					return $this->step_in_select();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_IN_SELECT_IN_TABLE:
+					return $this->step_in_select_in_table();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_IN_TEMPLATE:
+					return $this->step_in_template();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_AFTER_BODY:
+					return $this->step_after_body();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_IN_FRAMESET:
+					return $this->step_in_frameset();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_AFTER_FRAMESET:
+					return $this->step_after_frameset();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_AFTER_AFTER_BODY:
+					return $this->step_after_after_body();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_AFTER_AFTER_FRAMESET:
+					return $this->step_after_after_frameset();
+
+				// This should be unreachable but PHP doesn't have total type checking on switch.
 				default:
-					$this->last_error = self::ERROR_UNSUPPORTED;
-					throw new WP_HTML_Unsupported_Exception( "No support for parsing in the '{$this->state->insertion_mode}' state." );
+					$this->bail( "Unaware of the requested parsing mode: '{$this->state->insertion_mode}'." );
 			}
 		} catch ( WP_HTML_Unsupported_Exception $e ) {
 			/*
@@ -776,11 +1143,7 @@
 	 * Breadcrumbs start at the outermost parent and descend toward the matched element.
 	 * They always include the entire path from the root HTML node to the matched element.
 	 *
-	 * @todo It could be more efficient to expose a generator-based version of this function
-	 *       to avoid creating the array copy on tag iteration. If this is done, it would likely
-	 *       be more useful to walk up the stack when yielding instead of starting at the top.
-	 *
-	 * Example
+	 * Example:
 	 *
 	 *     $processor = WP_HTML_Processor::create_fragment( '<p><strong><em><img></em></strong></p>' );
 	 *     $processor->next_tag( 'IMG' );
@@ -788,49 +1151,10 @@
 	 *
 	 * @since 6.4.0
 	 *
-	 * @return string[]|null Array of tag names representing path to matched node, if matched, otherwise NULL.
+	 * @return string[] Array of tag names representing path to matched node.
 	 */
-	public function get_breadcrumbs() {
-		$breadcrumbs = array();
-
-		foreach ( $this->state->stack_of_open_elements->walk_down() as $stack_item ) {
-			$breadcrumbs[] = $stack_item->node_name;
-		}
-
-		if ( ! $this->is_virtual() ) {
-			return $breadcrumbs;
-		}
-
-		foreach ( $this->element_queue as $queue_item ) {
-			if ( $this->current_element->token->bookmark_name === $queue_item->token->bookmark_name ) {
-				break;
-			}
-
-			if ( 'context-node' === $queue_item->token->bookmark_name ) {
-				break;
-			}
-
-			if ( 'real' === $queue_item->provenance ) {
-				break;
-			}
-
-			if ( WP_HTML_Stack_Event::PUSH === $queue_item->operation ) {
-				$breadcrumbs[] = $queue_item->token->node_name;
-			} else {
-				array_pop( $breadcrumbs );
-			}
-		}
-
-		if ( null !== parent::get_token_name() && ! parent::is_tag_closer() ) {
-			array_pop( $breadcrumbs );
-		}
-
-		// Add the virtual node we're at.
-		if ( WP_HTML_Stack_Event::PUSH === $this->current_element->operation ) {
-			$breadcrumbs[] = $this->current_element->token->node_name;
-		}
-
-		return $breadcrumbs;
+	public function get_breadcrumbs(): array {
+		return $this->breadcrumbs;
 	}
 
 	/**
@@ -858,10 +1182,971 @@
 	 *
 	 * @return int Nesting-depth of current location in the document.
 	 */
-	public function get_current_depth() {
-		return $this->is_virtual()
-			? count( $this->get_breadcrumbs() )
-			: $this->state->stack_of_open_elements->count();
+	public function get_current_depth(): int {
+		return count( $this->breadcrumbs );
+	}
+
+	/**
+	 * Normalizes an HTML fragment by serializing it.
+	 *
+	 * This method assumes that the given HTML snippet is found in BODY context.
+	 * For normalizing full documents or fragments found in other contexts, create
+	 * a new processor using {@see WP_HTML_Processor::create_fragment} or
+	 * {@see WP_HTML_Processor::create_full_parser} and call {@see WP_HTML_Processor::serialize}
+	 * on the created instances.
+	 *
+	 * Many aspects of an input HTML fragment may be changed during normalization.
+	 *
+	 *  - Attribute values will be double-quoted.
+	 *  - Duplicate attributes will be removed.
+	 *  - Omitted tags will be added.
+	 *  - Tag and attribute name casing will be lower-cased,
+	 *    except for specific SVG and MathML tags or attributes.
+	 *  - Text will be re-encoded, null bytes handled,
+	 *    and invalid UTF-8 replaced with U+FFFD.
+	 *  - Any incomplete syntax trailing at the end will be omitted,
+	 *    for example, an unclosed comment opener will be removed.
+	 *
+	 * Example:
+	 *
+	 *     echo WP_HTML_Processor::normalize( '<a href=#anchor v=5 href="/" enabled>One</a another v=5><!--' );
+	 *     // <a href="#anchor" v="5" enabled>One</a>
+	 *
+	 *     echo WP_HTML_Processor::normalize( '<div></p>fun<table><td>cell</div>' );
+	 *     // <div><p></p>fun<table><tbody><tr><td>cell</td></tr></tbody></table></div>
+	 *
+	 *     echo WP_HTML_Processor::normalize( '<![CDATA[invalid comment]]> syntax < <> "oddities"' );
+	 *     // <!--[CDATA[invalid comment]]--> syntax &lt; &lt;&gt; &quot;oddities&quot;
+	 *
+	 * @since 6.7.0
+	 *
+	 * @param string $html Input HTML to normalize.
+	 *
+	 * @return string|null Normalized output, or `null` if unable to normalize.
+	 */
+	public static function normalize( string $html ): ?string {
+		return static::create_fragment( $html )->serialize();
+	}
+
+	/**
+	 * Returns normalized HTML for a fragment by serializing it.
+	 *
+	 * This differs from {@see WP_HTML_Processor::normalize} in that it starts with
+	 * a specific HTML Processor, which _must_ not have already started scanning;
+	 * it must be in the initial ready state and will be in the completed state once
+	 * serialization is complete.
+	 *
+	 * Many aspects of an input HTML fragment may be changed during normalization.
+	 *
+	 *  - Attribute values will be double-quoted.
+	 *  - Duplicate attributes will be removed.
+	 *  - Omitted tags will be added.
+	 *  - Tag and attribute name casing will be lower-cased,
+	 *    except for specific SVG and MathML tags or attributes.
+	 *  - Text will be re-encoded, null bytes handled,
+	 *    and invalid UTF-8 replaced with U+FFFD.
+	 *  - Any incomplete syntax trailing at the end will be omitted,
+	 *    for example, an unclosed comment opener will be removed.
+	 *
+	 * Example:
+	 *
+	 *     $processor = WP_HTML_Processor::create_fragment( '<a href=#anchor v=5 href="/" enabled>One</a another v=5><!--' );
+	 *     echo $processor->serialize();
+	 *     // <a href="#anchor" v="5" enabled>One</a>
+	 *
+	 *     $processor = WP_HTML_Processor::create_fragment( '<div></p>fun<table><td>cell</div>' );
+	 *     echo $processor->serialize();
+	 *     // <div><p></p>fun<table><tbody><tr><td>cell</td></tr></tbody></table></div>
+	 *
+	 *     $processor = WP_HTML_Processor::create_fragment( '<![CDATA[invalid comment]]> syntax < <> "oddities"' );
+	 *     echo $processor->serialize();
+	 *     // <!--[CDATA[invalid comment]]--> syntax &lt; &lt;&gt; &quot;oddities&quot;
+	 *
+	 * @since 6.7.0
+	 *
+	 * @return string|null Normalized HTML markup represented by processor,
+	 *                     or `null` if unable to generate serialization.
+	 */
+	public function serialize(): ?string {
+		if ( WP_HTML_Tag_Processor::STATE_READY !== $this->parser_state ) {
+			wp_trigger_error(
+				__METHOD__,
+				'An HTML Processor which has already started processing cannot serialize its contents. Serialize immediately after creating the instance.',
+				E_USER_WARNING
+			);
+			return null;
+		}
+
+		$html = '';
+		while ( $this->next_token() ) {
+			$html .= $this->serialize_token();
+		}
+
+		if ( null !== $this->get_last_error() ) {
+			wp_trigger_error(
+				__METHOD__,
+				"Cannot serialize HTML Processor with parsing error: {$this->get_last_error()}.",
+				E_USER_WARNING
+			);
+			return null;
+		}
+
+		return $html;
+	}
+
+	/**
+	 * Serializes the currently-matched token.
+	 *
+	 * This method produces a fully-normative HTML string for the currently-matched token,
+	 * if able. If not matched at any token or if the token doesn't correspond to any HTML
+	 * it will return an empty string (for example, presumptuous end tags are ignored).
+	 *
+	 * @see static::serialize()
+	 *
+	 * @since 6.7.0
+	 *
+	 * @return string Serialization of token, or empty string if no serialization exists.
+	 */
+	protected function serialize_token(): string {
+		$html       = '';
+		$token_type = $this->get_token_type();
+
+		switch ( $token_type ) {
+			case '#doctype':
+				$doctype = $this->get_doctype_info();
+				if ( null === $doctype ) {
+					break;
+				}
+
+				$html .= '<!DOCTYPE';
+
+				if ( $doctype->name ) {
+					$html .= " {$doctype->name}";
+				}
+
+				if ( null !== $doctype->public_identifier ) {
+					$quote = str_contains( $doctype->public_identifier, '"' ) ? "'" : '"';
+					$html .= " PUBLIC {$quote}{$doctype->public_identifier}{$quote}";
+				}
+				if ( null !== $doctype->system_identifier ) {
+					if ( null === $doctype->public_identifier ) {
+						$html .= ' SYSTEM';
+					}
+					$quote = str_contains( $doctype->system_identifier, '"' ) ? "'" : '"';
+					$html .= " {$quote}{$doctype->system_identifier}{$quote}";
+				}
+
+				$html .= '>';
+				break;
+
+			case '#text':
+				$html .= htmlspecialchars( $this->get_modifiable_text(), ENT_QUOTES | ENT_SUBSTITUTE | ENT_HTML5, 'UTF-8' );
+				break;
+
+			// Unlike the `<>` which is interpreted as plaintext, this is ignored entirely.
+			case '#presumptuous-tag':
+				break;
+
+			case '#funky-comment':
+			case '#comment':
+				$html .= "<!--{$this->get_full_comment_text()}-->";
+				break;
+
+			case '#cdata-section':
+				$html .= "<![CDATA[{$this->get_modifiable_text()}]]>";
+				break;
+		}
+
+		if ( '#tag' !== $token_type ) {
+			return $html;
+		}
+
+		$tag_name       = str_replace( "\x00", "\u{FFFD}", $this->get_tag() );
+		$in_html        = 'html' === $this->get_namespace();
+		$qualified_name = $in_html ? strtolower( $tag_name ) : $this->get_qualified_tag_name();
+
+		if ( $this->is_tag_closer() ) {
+			$html .= "</{$qualified_name}>";
+			return $html;
+		}
+
+		$attribute_names = $this->get_attribute_names_with_prefix( '' );
+		if ( ! isset( $attribute_names ) ) {
+			$html .= "<{$qualified_name}>";
+			return $html;
+		}
+
+		$html .= "<{$qualified_name}";
+		foreach ( $attribute_names as $attribute_name ) {
+			$html .= " {$this->get_qualified_attribute_name( $attribute_name )}";
+			$value = $this->get_attribute( $attribute_name );
+
+			if ( is_string( $value ) ) {
+				$html .= '="' . htmlspecialchars( $value, ENT_QUOTES | ENT_SUBSTITUTE | ENT_HTML5 ) . '"';
+			}
+
+			$html = str_replace( "\x00", "\u{FFFD}", $html );
+		}
+
+		if ( ! $in_html && $this->has_self_closing_flag() ) {
+			$html .= ' /';
+		}
+
+		$html .= '>';
+
+		// Flush out self-contained elements.
+		if ( $in_html && in_array( $tag_name, array( 'IFRAME', 'NOEMBED', 'NOFRAMES', 'SCRIPT', 'STYLE', 'TEXTAREA', 'TITLE', 'XMP' ), true ) ) {
+			$text = $this->get_modifiable_text();
+
+			switch ( $tag_name ) {
+				case 'IFRAME':
+				case 'NOEMBED':
+				case 'NOFRAMES':
+					$text = '';
+					break;
+
+				case 'SCRIPT':
+				case 'STYLE':
+					break;
+
+				default:
+					$text = htmlspecialchars( $text, ENT_QUOTES | ENT_SUBSTITUTE | ENT_HTML5, 'UTF-8' );
+			}
+
+			$html .= "{$text}</{$qualified_name}>";
+		}
+
+		return $html;
+	}
+
+	/**
+	 * Parses next element in the 'initial' insertion mode.
+	 *
+	 * This internal function performs the 'initial' insertion mode
+	 * logic for the generalized WP_HTML_Processor::step() function.
+	 *
+	 * @since 6.7.0
+	 *
+	 * @throws WP_HTML_Unsupported_Exception When encountering unsupported HTML input.
+	 *
+	 * @see https://html.spec.whatwg.org/#the-initial-insertion-mode
+	 * @see WP_HTML_Processor::step
+	 *
+	 * @return bool Whether an element was found.
+	 */
+	private function step_initial(): bool {
+		$token_name = $this->get_token_name();
+		$token_type = $this->get_token_type();
+		$op_sigil   = '#tag' === $token_type ? ( parent::is_tag_closer() ? '-' : '+' ) : '';
+		$op         = "{$op_sigil}{$token_name}";
+
+		switch ( $op ) {
+			/*
+			 * > A character token that is one of U+0009 CHARACTER TABULATION,
+			 * > U+000A LINE FEED (LF), U+000C FORM FEED (FF),
+			 * > U+000D CARRIAGE RETURN (CR), or U+0020 SPACE
+			 *
+			 * Parse error: ignore the token.
+			 */
+			case '#text':
+				if ( parent::TEXT_IS_WHITESPACE === $this->text_node_classification ) {
+					return $this->step();
+				}
+				goto initial_anything_else;
+				break;
+
+			/*
+			 * > A comment token
+			 */
+			case '#comment':
+			case '#funky-comment':
+			case '#presumptuous-tag':
+				$this->insert_html_element( $this->state->current_token );
+				return true;
+
+			/*
+			 * > A DOCTYPE token
+			 */
+			case 'html':
+				$doctype = $this->get_doctype_info();
+				if ( null !== $doctype && 'quirks' === $doctype->indicated_compatability_mode ) {
+					$this->compat_mode = WP_HTML_Tag_Processor::QUIRKS_MODE;
+				}
+
+				/*
+				 * > Then, switch the insertion mode to "before html".
+				 */
+				$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_BEFORE_HTML;
+				$this->insert_html_element( $this->state->current_token );
+				return true;
+		}
+
+		/*
+		 * > Anything else
+		 */
+		initial_anything_else:
+		$this->compat_mode           = WP_HTML_Tag_Processor::QUIRKS_MODE;
+		$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_BEFORE_HTML;
+		return $this->step( self::REPROCESS_CURRENT_NODE );
+	}
+
+	/**
+	 * Parses next element in the 'before html' insertion mode.
+	 *
+	 * This internal function performs the 'before html' insertion mode
+	 * logic for the generalized WP_HTML_Processor::step() function.
+	 *
+	 * @since 6.7.0
+	 *
+	 * @throws WP_HTML_Unsupported_Exception When encountering unsupported HTML input.
+	 *
+	 * @see https://html.spec.whatwg.org/#the-before-html-insertion-mode
+	 * @see WP_HTML_Processor::step
+	 *
+	 * @return bool Whether an element was found.
+	 */
+	private function step_before_html(): bool {
+		$token_name = $this->get_token_name();
+		$token_type = $this->get_token_type();
+		$is_closer  = parent::is_tag_closer();
+		$op_sigil   = '#tag' === $token_type ? ( $is_closer ? '-' : '+' ) : '';
+		$op         = "{$op_sigil}{$token_name}";
+
+		switch ( $op ) {
+			/*
+			 * > A DOCTYPE token
+			 */
+			case 'html':
+				// Parse error: ignore the token.
+				return $this->step();
+
+			/*
+			 * > A comment token
+			 */
+			case '#comment':
+			case '#funky-comment':
+			case '#presumptuous-tag':
+				$this->insert_html_element( $this->state->current_token );
+				return true;
+
+			/*
+			 * > A character token that is one of U+0009 CHARACTER TABULATION,
+			 * > U+000A LINE FEED (LF), U+000C FORM FEED (FF),
+			 * > U+000D CARRIAGE RETURN (CR), or U+0020 SPACE
+			 *
+			 * Parse error: ignore the token.
+			 */
+			case '#text':
+				if ( parent::TEXT_IS_WHITESPACE === $this->text_node_classification ) {
+					return $this->step();
+				}
+				goto before_html_anything_else;
+				break;
+
+			/*
+			 * > A start tag whose tag name is "html"
+			 */
+			case '+HTML':
+				$this->insert_html_element( $this->state->current_token );
+				$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_BEFORE_HEAD;
+				return true;
+
+			/*
+			 * > An end tag whose tag name is one of: "head", "body", "html", "br"
+			 *
+			 * Closing BR tags are always reported by the Tag Processor as opening tags.
+			 */
+			case '-HEAD':
+			case '-BODY':
+			case '-HTML':
+				/*
+				 * > Act as described in the "anything else" entry below.
+				 */
+				goto before_html_anything_else;
+				break;
+		}
+
+		/*
+		 * > Any other end tag
+		 */
+		if ( $is_closer ) {
+			// Parse error: ignore the token.
+			return $this->step();
+		}
+
+		/*
+		 * > Anything else.
+		 *
+		 * > Create an html element whose node document is the Document object.
+		 * > Append it to the Document object. Put this element in the stack of open elements.
+		 * > Switch the insertion mode to "before head", then reprocess the token.
+		 */
+		before_html_anything_else:
+		$this->insert_virtual_node( 'HTML' );
+		$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_BEFORE_HEAD;
+		return $this->step( self::REPROCESS_CURRENT_NODE );
+	}
+
+	/**
+	 * Parses next element in the 'before head' insertion mode.
+	 *
+	 * This internal function performs the 'before head' insertion mode
+	 * logic for the generalized WP_HTML_Processor::step() function.
+	 *
+	 * @since 6.7.0 Stub implementation.
+	 *
+	 * @throws WP_HTML_Unsupported_Exception When encountering unsupported HTML input.
+	 *
+	 * @see https://html.spec.whatwg.org/#the-before-head-insertion-mode
+	 * @see WP_HTML_Processor::step
+	 *
+	 * @return bool Whether an element was found.
+	 */
+	private function step_before_head(): bool {
+		$token_name = $this->get_token_name();
+		$token_type = $this->get_token_type();
+		$is_closer  = parent::is_tag_closer();
+		$op_sigil   = '#tag' === $token_type ? ( $is_closer ? '-' : '+' ) : '';
+		$op         = "{$op_sigil}{$token_name}";
+
+		switch ( $op ) {
+			/*
+			 * > A character token that is one of U+0009 CHARACTER TABULATION,
+			 * > U+000A LINE FEED (LF), U+000C FORM FEED (FF),
+			 * > U+000D CARRIAGE RETURN (CR), or U+0020 SPACE
+			 *
+			 * Parse error: ignore the token.
+			 */
+			case '#text':
+				if ( parent::TEXT_IS_WHITESPACE === $this->text_node_classification ) {
+					return $this->step();
+				}
+				goto before_head_anything_else;
+				break;
+
+			/*
+			 * > A comment token
+			 */
+			case '#comment':
+			case '#funky-comment':
+			case '#presumptuous-tag':
+				$this->insert_html_element( $this->state->current_token );
+				return true;
+
+			/*
+			 * > A DOCTYPE token
+			 */
+			case 'html':
+				// Parse error: ignore the token.
+				return $this->step();
+
+			/*
+			 * > A start tag whose tag name is "html"
+			 */
+			case '+HTML':
+				return $this->step_in_body();
+
+			/*
+			 * > A start tag whose tag name is "head"
+			 */
+			case '+HEAD':
+				$this->insert_html_element( $this->state->current_token );
+				$this->state->head_element   = $this->state->current_token;
+				$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_HEAD;
+				return true;
+
+			/*
+			 * > An end tag whose tag name is one of: "head", "body", "html", "br"
+			 * > Act as described in the "anything else" entry below.
+			 *
+			 * Closing BR tags are always reported by the Tag Processor as opening tags.
+			 */
+			case '-HEAD':
+			case '-BODY':
+			case '-HTML':
+				goto before_head_anything_else;
+				break;
+		}
+
+		if ( $is_closer ) {
+			// Parse error: ignore the token.
+			return $this->step();
+		}
+
+		/*
+		 * > Anything else
+		 *
+		 * > Insert an HTML element for a "head" start tag token with no attributes.
+		 */
+		before_head_anything_else:
+		$this->state->head_element   = $this->insert_virtual_node( 'HEAD' );
+		$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_HEAD;
+		return $this->step( self::REPROCESS_CURRENT_NODE );
+	}
+
+	/**
+	 * Parses next element in the 'in head' insertion mode.
+	 *
+	 * This internal function performs the 'in head' insertion mode
+	 * logic for the generalized WP_HTML_Processor::step() function.
+	 *
+	 * @since 6.7.0
+	 *
+	 * @throws WP_HTML_Unsupported_Exception When encountering unsupported HTML input.
+	 *
+	 * @see https://html.spec.whatwg.org/multipage/parsing.html#parsing-main-inhead
+	 * @see WP_HTML_Processor::step
+	 *
+	 * @return bool Whether an element was found.
+	 */
+	private function step_in_head(): bool {
+		$token_name = $this->get_token_name();
+		$token_type = $this->get_token_type();
+		$is_closer  = parent::is_tag_closer();
+		$op_sigil   = '#tag' === $token_type ? ( $is_closer ? '-' : '+' ) : '';
+		$op         = "{$op_sigil}{$token_name}";
+
+		switch ( $op ) {
+			case '#text':
+				/*
+				 * > A character token that is one of U+0009 CHARACTER TABULATION,
+				 * > U+000A LINE FEED (LF), U+000C FORM FEED (FF),
+				 * > U+000D CARRIAGE RETURN (CR), or U+0020 SPACE
+				 */
+				if ( parent::TEXT_IS_WHITESPACE === $this->text_node_classification ) {
+					// Insert the character.
+					$this->insert_html_element( $this->state->current_token );
+					return true;
+				}
+
+				goto in_head_anything_else;
+				break;
+
+			/*
+			 * > A comment token
+			 */
+			case '#comment':
+			case '#funky-comment':
+			case '#presumptuous-tag':
+				$this->insert_html_element( $this->state->current_token );
+				return true;
+
+			/*
+			 * > A DOCTYPE token
+			 */
+			case 'html':
+				// Parse error: ignore the token.
+				return $this->step();
+
+			/*
+			 * > A start tag whose tag name is "html"
+			 */
+			case '+HTML':
+				return $this->step_in_body();
+
+			/*
+			 * > A start tag whose tag name is one of: "base", "basefont", "bgsound", "link"
+			 */
+			case '+BASE':
+			case '+BASEFONT':
+			case '+BGSOUND':
+			case '+LINK':
+				$this->insert_html_element( $this->state->current_token );
+				return true;
+
+			/*
+			 * > A start tag whose tag name is "meta"
+			 */
+			case '+META':
+				$this->insert_html_element( $this->state->current_token );
+
+				/*
+				 * > If the active speculative HTML parser is null, then:
+				 * >   - If the element has a charset attribute, and getting an encoding from
+				 * >     its value results in an encoding, and the confidence is currently
+				 * >     tentative, then change the encoding to the resulting encoding.
+				 */
+				$charset = $this->get_attribute( 'charset' );
+				if ( is_string( $charset ) && 'tentative' === $this->state->encoding_confidence ) {
+					$this->bail( 'Cannot yet process META tags with charset to determine encoding.' );
+				}
+
+				/*
+				 * >   - Otherwise, if the element has an http-equiv attribute whose value is
+				 * >     an ASCII case-insensitive match for the string "Content-Type", and
+				 * >     the element has a content attribute, and applying the algorithm for
+				 * >     extracting a character encoding from a meta element to that attribute's
+				 * >     value returns an encoding, and the confidence is currently tentative,
+				 * >     then change the encoding to the extracted encoding.
+				 */
+				$http_equiv = $this->get_attribute( 'http-equiv' );
+				$content    = $this->get_attribute( 'content' );
+				if (
+					is_string( $http_equiv ) &&
+					is_string( $content ) &&
+					0 === strcasecmp( $http_equiv, 'Content-Type' ) &&
+					'tentative' === $this->state->encoding_confidence
+				) {
+					$this->bail( 'Cannot yet process META tags with http-equiv Content-Type to determine encoding.' );
+				}
+
+				return true;
+
+			/*
+			 * > A start tag whose tag name is "title"
+			 */
+			case '+TITLE':
+				$this->insert_html_element( $this->state->current_token );
+				return true;
+
+			/*
+			 * > A start tag whose tag name is "noscript", if the scripting flag is enabled
+			 * > A start tag whose tag name is one of: "noframes", "style"
+			 *
+			 * The scripting flag is never enabled in this parser.
+			 */
+			case '+NOFRAMES':
+			case '+STYLE':
+				$this->insert_html_element( $this->state->current_token );
+				return true;
+
+			/*
+			 * > A start tag whose tag name is "noscript", if the scripting flag is disabled
+			 */
+			case '+NOSCRIPT':
+				$this->insert_html_element( $this->state->current_token );
+				$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_HEAD_NOSCRIPT;
+				return true;
+
+			/*
+			 * > A start tag whose tag name is "script"
+			 *
+			 * @todo Could the adjusted insertion location be anything other than the current location?
+			 */
+			case '+SCRIPT':
+				$this->insert_html_element( $this->state->current_token );
+				return true;
+
+			/*
+			 * > An end tag whose tag name is "head"
+			 */
+			case '-HEAD':
+				$this->state->stack_of_open_elements->pop();
+				$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_AFTER_HEAD;
+				return true;
+
+			/*
+			 * > An end tag whose tag name is one of: "body", "html", "br"
+			 *
+			 * BR tags are always reported by the Tag Processor as opening tags.
+			 */
+			case '-BODY':
+			case '-HTML':
+				/*
+				 * > Act as described in the "anything else" entry below.
+				 */
+				goto in_head_anything_else;
+				break;
+
+			/*
+			 * > A start tag whose tag name is "template"
+			 *
+			 * @todo Could the adjusted insertion location be anything other than the current location?
+			 */
+			case '+TEMPLATE':
+				$this->state->active_formatting_elements->insert_marker();
+				$this->state->frameset_ok = false;
+
+				$this->state->insertion_mode                      = WP_HTML_Processor_State::INSERTION_MODE_IN_TEMPLATE;
+				$this->state->stack_of_template_insertion_modes[] = WP_HTML_Processor_State::INSERTION_MODE_IN_TEMPLATE;
+
+				$this->insert_html_element( $this->state->current_token );
+				return true;
+
+			/*
+			 * > An end tag whose tag name is "template"
+			 */
+			case '-TEMPLATE':
+				if ( ! $this->state->stack_of_open_elements->contains( 'TEMPLATE' ) ) {
+					// @todo Indicate a parse error once it's possible.
+					return $this->step();
+				}
+
+				$this->generate_implied_end_tags_thoroughly();
+				if ( ! $this->state->stack_of_open_elements->current_node_is( 'TEMPLATE' ) ) {
+					// @todo Indicate a parse error once it's possible.
+				}
+
+				$this->state->stack_of_open_elements->pop_until( 'TEMPLATE' );
+				$this->state->active_formatting_elements->clear_up_to_last_marker();
+				array_pop( $this->state->stack_of_template_insertion_modes );
+				$this->reset_insertion_mode_appropriately();
+				return true;
+		}
+
+		/*
+		 * > A start tag whose tag name is "head"
+		 * > Any other end tag
+		 */
+		if ( '+HEAD' === $op || $is_closer ) {
+			// Parse error: ignore the token.
+			return $this->step();
+		}
+
+		/*
+		 * > Anything else
+		 */
+		in_head_anything_else:
+		$this->state->stack_of_open_elements->pop();
+		$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_AFTER_HEAD;
+		return $this->step( self::REPROCESS_CURRENT_NODE );
+	}
+
+	/**
+	 * Parses next element in the 'in head noscript' insertion mode.
+	 *
+	 * This internal function performs the 'in head noscript' insertion mode
+	 * logic for the generalized WP_HTML_Processor::step() function.
+	 *
+	 * @since 6.7.0 Stub implementation.
+	 *
+	 * @throws WP_HTML_Unsupported_Exception When encountering unsupported HTML input.
+	 *
+	 * @see https://html.spec.whatwg.org/#parsing-main-inheadnoscript
+	 * @see WP_HTML_Processor::step
+	 *
+	 * @return bool Whether an element was found.
+	 */
+	private function step_in_head_noscript(): bool {
+		$token_name = $this->get_token_name();
+		$token_type = $this->get_token_type();
+		$is_closer  = parent::is_tag_closer();
+		$op_sigil   = '#tag' === $token_type ? ( $is_closer ? '-' : '+' ) : '';
+		$op         = "{$op_sigil}{$token_name}";
+
+		switch ( $op ) {
+			/*
+			 * > A character token that is one of U+0009 CHARACTER TABULATION,
+			 * > U+000A LINE FEED (LF), U+000C FORM FEED (FF),
+			 * > U+000D CARRIAGE RETURN (CR), or U+0020 SPACE
+			 *
+			 * Parse error: ignore the token.
+			 */
+			case '#text':
+				if ( parent::TEXT_IS_WHITESPACE === $this->text_node_classification ) {
+					return $this->step_in_head();
+				}
+
+				goto in_head_noscript_anything_else;
+				break;
+
+			/*
+			 * > A DOCTYPE token
+			 */
+			case 'html':
+				// Parse error: ignore the token.
+				return $this->step();
+
+			/*
+			 * > A start tag whose tag name is "html"
+			 */
+			case '+HTML':
+				return $this->step_in_body();
+
+			/*
+			 * > An end tag whose tag name is "noscript"
+			 */
+			case '-NOSCRIPT':
+				$this->state->stack_of_open_elements->pop();
+				$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_HEAD;
+				return true;
+
+			/*
+			 * > A comment token
+			 * >
+			 * > A start tag whose tag name is one of: "basefont", "bgsound",
+			 * > "link", "meta", "noframes", "style"
+			 */
+			case '#comment':
+			case '#funky-comment':
+			case '#presumptuous-tag':
+			case '+BASEFONT':
+			case '+BGSOUND':
+			case '+LINK':
+			case '+META':
+			case '+NOFRAMES':
+			case '+STYLE':
+				return $this->step_in_head();
+
+			/*
+			 * > An end tag whose tag name is "br"
+			 *
+			 * This should never happen, as the Tag Processor prevents showing a BR closing tag.
+			 */
+		}
+
+		/*
+		 * > A start tag whose tag name is one of: "head", "noscript"
+		 * > Any other end tag
+		 */
+		if ( '+HEAD' === $op || '+NOSCRIPT' === $op || $is_closer ) {
+			// Parse error: ignore the token.
+			return $this->step();
+		}
+
+		/*
+		 * > Anything else
+		 *
+		 * Anything here is a parse error.
+		 */
+		in_head_noscript_anything_else:
+		$this->state->stack_of_open_elements->pop();
+		$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_HEAD;
+		return $this->step( self::REPROCESS_CURRENT_NODE );
+	}
+
+	/**
+	 * Parses next element in the 'after head' insertion mode.
+	 *
+	 * This internal function performs the 'after head' insertion mode
+	 * logic for the generalized WP_HTML_Processor::step() function.
+	 *
+	 * @since 6.7.0 Stub implementation.
+	 *
+	 * @throws WP_HTML_Unsupported_Exception When encountering unsupported HTML input.
+	 *
+	 * @see https://html.spec.whatwg.org/#the-after-head-insertion-mode
+	 * @see WP_HTML_Processor::step
+	 *
+	 * @return bool Whether an element was found.
+	 */
+	private function step_after_head(): bool {
+		$token_name = $this->get_token_name();
+		$token_type = $this->get_token_type();
+		$is_closer  = parent::is_tag_closer();
+		$op_sigil   = '#tag' === $token_type ? ( $is_closer ? '-' : '+' ) : '';
+		$op         = "{$op_sigil}{$token_name}";
+
+		switch ( $op ) {
+			/*
+			 * > A character token that is one of U+0009 CHARACTER TABULATION,
+			 * > U+000A LINE FEED (LF), U+000C FORM FEED (FF),
+			 * > U+000D CARRIAGE RETURN (CR), or U+0020 SPACE
+			 */
+			case '#text':
+				if ( parent::TEXT_IS_WHITESPACE === $this->text_node_classification ) {
+					// Insert the character.
+					$this->insert_html_element( $this->state->current_token );
+					return true;
+				}
+				goto after_head_anything_else;
+				break;
+
+			/*
+			 * > A comment token
+			 */
+			case '#comment':
+			case '#funky-comment':
+			case '#presumptuous-tag':
+				$this->insert_html_element( $this->state->current_token );
+				return true;
+
+			/*
+			 * > A DOCTYPE token
+			 */
+			case 'html':
+				// Parse error: ignore the token.
+				return $this->step();
+
+			/*
+			 * > A start tag whose tag name is "html"
+			 */
+			case '+HTML':
+				return $this->step_in_body();
+
+			/*
+			 * > A start tag whose tag name is "body"
+			 */
+			case '+BODY':
+				$this->insert_html_element( $this->state->current_token );
+				$this->state->frameset_ok    = false;
+				$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_BODY;
+				return true;
+
+			/*
+			 * > A start tag whose tag name is "frameset"
+			 */
+			case '+FRAMESET':
+				$this->insert_html_element( $this->state->current_token );
+				$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_FRAMESET;
+				return true;
+
+			/*
+			 * > A start tag whose tag name is one of: "base", "basefont", "bgsound",
+			 * > "link", "meta", "noframes", "script", "style", "template", "title"
+			 *
+			 * Anything here is a parse error.
+			 */
+			case '+BASE':
+			case '+BASEFONT':
+			case '+BGSOUND':
+			case '+LINK':
+			case '+META':
+			case '+NOFRAMES':
+			case '+SCRIPT':
+			case '+STYLE':
+			case '+TEMPLATE':
+			case '+TITLE':
+				/*
+				 * > Push the node pointed to by the head element pointer onto the stack of open elements.
+				 * > Process the token using the rules for the "in head" insertion mode.
+				 * > Remove the node pointed to by the head element pointer from the stack of open elements. (It might not be the current node at this point.)
+				 */
+				$this->bail( 'Cannot process elements after HEAD which reopen the HEAD element.' );
+				/*
+				 * Do not leave this break in when adding support; it's here to prevent
+				 * WPCS from getting confused at the switch structure without a return,
+				 * because it doesn't know that `bail()` always throws.
+				 */
+				break;
+
+			/*
+			 * > An end tag whose tag name is "template"
+			 */
+			case '-TEMPLATE':
+				return $this->step_in_head();
+
+			/*
+			 * > An end tag whose tag name is one of: "body", "html", "br"
+			 *
+			 * Closing BR tags are always reported by the Tag Processor as opening tags.
+			 */
+			case '-BODY':
+			case '-HTML':
+				/*
+				 * > Act as described in the "anything else" entry below.
+				 */
+				goto after_head_anything_else;
+				break;
+		}
+
+		/*
+		 * > A start tag whose tag name is "head"
+		 * > Any other end tag
+		 */
+		if ( '+HEAD' === $op || $is_closer ) {
+			// Parse error: ignore the token.
+			return $this->step();
+		}
+
+		/*
+		 * > Anything else
+		 * > Insert an HTML element for a "body" start tag token with no attributes.
+		 */
+		after_head_anything_else:
+		$this->insert_virtual_node( 'BODY' );
+		$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_BODY;
+		return $this->step( self::REPROCESS_CURRENT_NODE );
 	}
 
 	/**
@@ -879,24 +2164,14 @@
 	 *
 	 * @return bool Whether an element was found.
 	 */
-	private function step_in_body() {
+	private function step_in_body(): bool {
 		$token_name = $this->get_token_name();
 		$token_type = $this->get_token_type();
 		$op_sigil   = '#tag' === $token_type ? ( parent::is_tag_closer() ? '-' : '+' ) : '';
 		$op         = "{$op_sigil}{$token_name}";
 
 		switch ( $op ) {
-			case '#comment':
-			case '#funky-comment':
-			case '#presumptuous-tag':
-				$this->insert_html_element( $this->state->current_token );
-				return true;
-
 			case '#text':
-				$this->reconstruct_active_formatting_elements();
-
-				$current_token = $this->bookmarks[ $this->state->current_token->bookmark_name ];
-
 				/*
 				 * > A character token that is U+0000 NULL
 				 *
@@ -906,50 +2181,171 @@
 				 * here, but if there are any other characters in the stream
 				 * the active formats should be reconstructed.
 				 */
-				if (
-					1 <= $current_token->length &&
-					"\x00" === $this->html[ $current_token->start ] &&
-					strspn( $this->html, "\x00", $current_token->start, $current_token->length ) === $current_token->length
-				) {
+				if ( parent::TEXT_IS_NULL_SEQUENCE === $this->text_node_classification ) {
 					// Parse error: ignore the token.
 					return $this->step();
 				}
 
+				$this->reconstruct_active_formatting_elements();
+
 				/*
 				 * Whitespace-only text does not affect the frameset-ok flag.
 				 * It is probably inter-element whitespace, but it may also
 				 * contain character references which decode only to whitespace.
 				 */
-				$text = $this->get_modifiable_text();
-				if ( strlen( $text ) !== strspn( $text, " \t\n\f\r" ) ) {
+				if ( parent::TEXT_IS_GENERIC === $this->text_node_classification ) {
 					$this->state->frameset_ok = false;
 				}
 
 				$this->insert_html_element( $this->state->current_token );
 				return true;
 
+			case '#comment':
+			case '#funky-comment':
+			case '#presumptuous-tag':
+				$this->insert_html_element( $this->state->current_token );
+				return true;
+
+			/*
+			 * > A DOCTYPE token
+			 * > Parse error. Ignore the token.
+			 */
 			case 'html':
+				return $this->step();
+
+			/*
+			 * > A start tag whose tag name is "html"
+			 */
+			case '+HTML':
+				if ( ! $this->state->stack_of_open_elements->contains( 'TEMPLATE' ) ) {
+					/*
+					 * > Otherwise, for each attribute on the token, check to see if the attribute
+					 * > is already present on the top element of the stack of open elements. If
+					 * > it is not, add the attribute and its corresponding value to that element.
+					 *
+					 * This parser does not currently support this behavior: ignore the token.
+					 */
+				}
+
+				// Ignore the token.
+				return $this->step();
+
+			/*
+			 * > A start tag whose tag name is one of: "base", "basefont", "bgsound", "link",
+			 * > "meta", "noframes", "script", "style", "template", "title"
+			 * >
+			 * > An end tag whose tag name is "template"
+			 */
+			case '+BASE':
+			case '+BASEFONT':
+			case '+BGSOUND':
+			case '+LINK':
+			case '+META':
+			case '+NOFRAMES':
+			case '+SCRIPT':
+			case '+STYLE':
+			case '+TEMPLATE':
+			case '+TITLE':
+			case '-TEMPLATE':
+				return $this->step_in_head();
+
+			/*
+			 * > A start tag whose tag name is "body"
+			 *
+			 * This tag in the IN BODY insertion mode is a parse error.
+			 */
+			case '+BODY':
+				if (
+					1 === $this->state->stack_of_open_elements->count() ||
+					'BODY' !== ( $this->state->stack_of_open_elements->at( 2 )->node_name ?? null ) ||
+					$this->state->stack_of_open_elements->contains( 'TEMPLATE' )
+				) {
+					// Ignore the token.
+					return $this->step();
+				}
+
 				/*
-				 * > A DOCTYPE token
-				 * > Parse error. Ignore the token.
+				 * > Otherwise, set the frameset-ok flag to "not ok"; then, for each attribute
+				 * > on the token, check to see if the attribute is already present on the body
+				 * > element (the second element) on the stack of open elements, and if it is
+				 * > not, add the attribute and its corresponding value to that element.
+				 *
+				 * This parser does not currently support this behavior: ignore the token.
+				 */
+				$this->state->frameset_ok = false;
+				return $this->step();
+
+			/*
+			 * > A start tag whose tag name is "frameset"
+			 *
+			 * This tag in the IN BODY insertion mode is a parse error.
+			 */
+			case '+FRAMESET':
+				if (
+					1 === $this->state->stack_of_open_elements->count() ||
+					'BODY' !== ( $this->state->stack_of_open_elements->at( 2 )->node_name ?? null ) ||
+					false === $this->state->frameset_ok
+				) {
+					// Ignore the token.
+					return $this->step();
+				}
+
+				/*
+				 * > Otherwise, run the following steps:
+				 */
+				$this->bail( 'Cannot process non-ignored FRAMESET tags.' );
+				break;
+
+			/*
+			 * > An end tag whose tag name is "body"
+			 */
+			case '-BODY':
+				if ( ! $this->state->stack_of_open_elements->has_element_in_scope( 'BODY' ) ) {
+					// Parse error: ignore the token.
+					return $this->step();
+				}
+
+				/*
+				 * > Otherwise, if there is a node in the stack of open elements that is not either a
+				 * > dd element, a dt element, an li element, an optgroup element, an option element,
+				 * > a p element, an rb element, an rp element, an rt element, an rtc element, a tbody
+				 * > element, a td element, a tfoot element, a th element, a thread element, a tr
+				 * > element, the body element, or the html element, then this is a parse error.
+				 *
+				 * There is nothing to do for this parse error, so don't check for it.
+				 */
+
+				$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_AFTER_BODY;
+				/*
+				 * The BODY element is not removed from the stack of open elements.
+				 * Only internal state has changed, this does not qualify as a "step"
+				 * in terms of advancing through the document to another token.
+				 * Nothing has been pushed or popped.
+				 * Proceed to parse the next item.
 				 */
 				return $this->step();
 
 			/*
-			 * > A start tag whose tag name is "button"
-			 */
-			case '+BUTTON':
-				if ( $this->state->stack_of_open_elements->has_element_in_scope( 'BUTTON' ) ) {
-					// @todo Indicate a parse error once it's possible. This error does not impact the logic here.
-					$this->generate_implied_end_tags();
-					$this->state->stack_of_open_elements->pop_until( 'BUTTON' );
+			 * > An end tag whose tag name is "html"
+			 */
+			case '-HTML':
+				if ( ! $this->state->stack_of_open_elements->has_element_in_scope( 'BODY' ) ) {
+					// Parse error: ignore the token.
+					return $this->step();
 				}
 
-				$this->reconstruct_active_formatting_elements();
-				$this->insert_html_element( $this->state->current_token );
-				$this->state->frameset_ok = false;
-
-				return true;
+				/*
+				 * > Otherwise, if there is a node in the stack of open elements that is not either a
+				 * > dd element, a dt element, an li element, an optgroup element, an option element,
+				 * > a p element, an rb element, an rp element, an rt element, an rtc element, a tbody
+				 * > element, a td element, a tfoot element, a th element, a thread element, a tr
+				 * > element, the body element, or the html element, then this is a parse error.
+				 *
+				 * There is nothing to do for this parse error, so don't check for it.
+				 */
+
+				$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_AFTER_BODY;
+				return $this->step( self::REPROCESS_CURRENT_NODE );
 
 			/*
 			 * > A start tag whose tag name is one of: "address", "article", "aside",
@@ -990,6 +2386,163 @@
 				return true;
 
 			/*
+			 * > A start tag whose tag name is one of: "h1", "h2", "h3", "h4", "h5", "h6"
+			 */
+			case '+H1':
+			case '+H2':
+			case '+H3':
+			case '+H4':
+			case '+H5':
+			case '+H6':
+				if ( $this->state->stack_of_open_elements->has_p_in_button_scope() ) {
+					$this->close_a_p_element();
+				}
+
+				if (
+					in_array(
+						$this->state->stack_of_open_elements->current_node()->node_name,
+						array( 'H1', 'H2', 'H3', 'H4', 'H5', 'H6' ),
+						true
+					)
+				) {
+					// @todo Indicate a parse error once it's possible.
+					$this->state->stack_of_open_elements->pop();
+				}
+
+				$this->insert_html_element( $this->state->current_token );
+				return true;
+
+			/*
+			 * > A start tag whose tag name is one of: "pre", "listing"
+			 */
+			case '+PRE':
+			case '+LISTING':
+				if ( $this->state->stack_of_open_elements->has_p_in_button_scope() ) {
+					$this->close_a_p_element();
+				}
+
+				/*
+				 * > If the next token is a U+000A LINE FEED (LF) character token,
+				 * > then ignore that token and move on to the next one. (Newlines
+				 * > at the start of pre blocks are ignored as an authoring convenience.)
+				 *
+				 * This is handled in `get_modifiable_text()`.
+				 */
+
+				$this->insert_html_element( $this->state->current_token );
+				$this->state->frameset_ok = false;
+				return true;
+
+			/*
+			 * > A start tag whose tag name is "form"
+			 */
+			case '+FORM':
+				$stack_contains_template = $this->state->stack_of_open_elements->contains( 'TEMPLATE' );
+
+				if ( isset( $this->state->form_element ) && ! $stack_contains_template ) {
+					// Parse error: ignore the token.
+					return $this->step();
+				}
+
+				if ( $this->state->stack_of_open_elements->has_p_in_button_scope() ) {
+					$this->close_a_p_element();
+				}
+
+				$this->insert_html_element( $this->state->current_token );
+				if ( ! $stack_contains_template ) {
+					$this->state->form_element = $this->state->current_token;
+				}
+
+				return true;
+
+			/*
+			 * > A start tag whose tag name is "li"
+			 * > A start tag whose tag name is one of: "dd", "dt"
+			 */
+			case '+DD':
+			case '+DT':
+			case '+LI':
+				$this->state->frameset_ok = false;
+				$node                     = $this->state->stack_of_open_elements->current_node();
+				$is_li                    = 'LI' === $token_name;
+
+				in_body_list_loop:
+				/*
+				 * The logic for LI and DT/DD is the same except for one point: LI elements _only_
+				 * close other LI elements, but a DT or DD element closes _any_ open DT or DD element.
+				 */
+				if ( $is_li ? 'LI' === $node->node_name : ( 'DD' === $node->node_name || 'DT' === $node->node_name ) ) {
+					$node_name = $is_li ? 'LI' : $node->node_name;
+					$this->generate_implied_end_tags( $node_name );
+					if ( ! $this->state->stack_of_open_elements->current_node_is( $node_name ) ) {
+						// @todo Indicate a parse error once it's possible. This error does not impact the logic here.
+					}
+
+					$this->state->stack_of_open_elements->pop_until( $node_name );
+					goto in_body_list_done;
+				}
+
+				if (
+					'ADDRESS' !== $node->node_name &&
+					'DIV' !== $node->node_name &&
+					'P' !== $node->node_name &&
+					self::is_special( $node )
+				) {
+					/*
+					 * > If node is in the special category, but is not an address, div,
+					 * > or p element, then jump to the step labeled done below.
+					 */
+					goto in_body_list_done;
+				} else {
+					/*
+					 * > Otherwise, set node to the previous entry in the stack of open elements
+					 * > and return to the step labeled loop.
+					 */
+					foreach ( $this->state->stack_of_open_elements->walk_up( $node ) as $item ) {
+						$node = $item;
+						break;
+					}
+					goto in_body_list_loop;
+				}
+
+				in_body_list_done:
+				if ( $this->state->stack_of_open_elements->has_p_in_button_scope() ) {
+					$this->close_a_p_element();
+				}
+
+				$this->insert_html_element( $this->state->current_token );
+				return true;
+
+			case '+PLAINTEXT':
+				if ( $this->state->stack_of_open_elements->has_p_in_button_scope() ) {
+					$this->close_a_p_element();
+				}
+
+				/*
+				 * @todo This may need to be handled in the Tag Processor and turn into
+				 *       a single self-contained tag like TEXTAREA, whose modifiable text
+				 *       is the rest of the input document as plaintext.
+				 */
+				$this->bail( 'Cannot process PLAINTEXT elements.' );
+				break;
+
+			/*
+			 * > A start tag whose tag name is "button"
+			 */
+			case '+BUTTON':
+				if ( $this->state->stack_of_open_elements->has_element_in_scope( 'BUTTON' ) ) {
+					// @todo Indicate a parse error once it's possible. This error does not impact the logic here.
+					$this->generate_implied_end_tags();
+					$this->state->stack_of_open_elements->pop_until( 'BUTTON' );
+				}
+
+				$this->reconstruct_active_formatting_elements();
+				$this->insert_html_element( $this->state->current_token );
+				$this->state->frameset_ok = false;
+
+				return true;
+
+			/*
 			 * > An end tag whose tag name is one of: "address", "article", "aside", "blockquote",
 			 * > "button", "center", "details", "dialog", "dir", "div", "dl", "fieldset",
 			 * > "figcaption", "figure", "footer", "header", "hgroup", "listing", "main",
@@ -1029,134 +2582,75 @@
 				}
 
 				$this->generate_implied_end_tags();
-				if ( $this->state->stack_of_open_elements->current_node()->node_name !== $token_name ) {
+				if ( ! $this->state->stack_of_open_elements->current_node_is( $token_name ) ) {
 					// @todo Record parse error: this error doesn't impact parsing.
 				}
 				$this->state->stack_of_open_elements->pop_until( $token_name );
 				return true;
 
 			/*
-			 * > A start tag whose tag name is one of: "h1", "h2", "h3", "h4", "h5", "h6"
-			 */
-			case '+H1':
-			case '+H2':
-			case '+H3':
-			case '+H4':
-			case '+H5':
-			case '+H6':
-				if ( $this->state->stack_of_open_elements->has_p_in_button_scope() ) {
-					$this->close_a_p_element();
-				}
-
-				if (
-					in_array(
-						$this->state->stack_of_open_elements->current_node()->node_name,
-						array( 'H1', 'H2', 'H3', 'H4', 'H5', 'H6' ),
-						true
-					)
-				) {
-					// @todo Indicate a parse error once it's possible.
-					$this->state->stack_of_open_elements->pop();
-				}
-
-				$this->insert_html_element( $this->state->current_token );
-				return true;
-
-			/*
-			 * > A start tag whose tag name is one of: "pre", "listing"
-			 */
-			case '+PRE':
-			case '+LISTING':
-				if ( $this->state->stack_of_open_elements->has_p_in_button_scope() ) {
-					$this->close_a_p_element();
-				}
-				$this->insert_html_element( $this->state->current_token );
-				$this->state->frameset_ok = false;
-				return true;
-
-			/*
-			 * > An end tag whose tag name is one of: "h1", "h2", "h3", "h4", "h5", "h6"
-			 */
-			case '-H1':
-			case '-H2':
-			case '-H3':
-			case '-H4':
-			case '-H5':
-			case '-H6':
-				if ( ! $this->state->stack_of_open_elements->has_element_in_scope( '(internal: H1 through H6 - do not use)' ) ) {
+			 * > An end tag whose tag name is "form"
+			 */
+			case '-FORM':
+				if ( ! $this->state->stack_of_open_elements->contains( 'TEMPLATE' ) ) {
+					$node                      = $this->state->form_element;
+					$this->state->form_element = null;
+
 					/*
-					 * This is a parse error; ignore the token.
+					 * > If node is null or if the stack of open elements does not have node
+					 * > in scope, then this is a parse error; return and ignore the token.
 					 *
-					 * @todo Indicate a parse error once it's possible.
+					 * @todo It's necessary to check if the form token itself is in scope, not
+					 *       simply whether any FORM is in scope.
 					 */
-					return $this->step();
-				}
-
-				$this->generate_implied_end_tags();
-
-				if ( $this->state->stack_of_open_elements->current_node()->node_name !== $token_name ) {
-					// @todo Record parse error: this error doesn't impact parsing.
-				}
-
-				$this->state->stack_of_open_elements->pop_until( '(internal: H1 through H6 - do not use)' );
-				return true;
-
-			/*
-			 * > A start tag whose tag name is "li"
-			 * > A start tag whose tag name is one of: "dd", "dt"
-			 */
-			case '+DD':
-			case '+DT':
-			case '+LI':
-				$this->state->frameset_ok = false;
-				$node                     = $this->state->stack_of_open_elements->current_node();
-				$is_li                    = 'LI' === $token_name;
-
-				in_body_list_loop:
-				/*
-				 * The logic for LI and DT/DD is the same except for one point: LI elements _only_
-				 * close other LI elements, but a DT or DD element closes _any_ open DT or DD element.
-				 */
-				if ( $is_li ? 'LI' === $node->node_name : ( 'DD' === $node->node_name || 'DT' === $node->node_name ) ) {
-					$node_name = $is_li ? 'LI' : $node->node_name;
-					$this->generate_implied_end_tags( $node_name );
-					if ( $node_name !== $this->state->stack_of_open_elements->current_node()->node_name ) {
+					if (
+						null === $node ||
+						! $this->state->stack_of_open_elements->has_element_in_scope( 'FORM' )
+					) {
+						// Parse error: ignore the token.
+						return $this->step();
+					}
+
+					$this->generate_implied_end_tags();
+					if ( $node !== $this->state->stack_of_open_elements->current_node() ) {
+						// @todo Indicate a parse error once it's possible. This error does not impact the logic here.
+						$this->bail( 'Cannot close a FORM when other elements remain open as this would throw off the breadcrumbs for the following tokens.' );
+					}
+
+					$this->state->stack_of_open_elements->remove_node( $node );
+					return true;
+				} else {
+					/*
+					 * > If the stack of open elements does not have a form element in scope,
+					 * > then this is a parse error; return and ignore the token.
+					 *
+					 * Note that unlike in the clause above, this is checking for any FORM in scope.
+					 */
+					if ( ! $this->state->stack_of_open_elements->has_element_in_scope( 'FORM' ) ) {
+						// Parse error: ignore the token.
+						return $this->step();
+					}
+
+					$this->generate_implied_end_tags();
+
+					if ( ! $this->state->stack_of_open_elements->current_node_is( 'FORM' ) ) {
 						// @todo Indicate a parse error once it's possible. This error does not impact the logic here.
 					}
 
-					$this->state->stack_of_open_elements->pop_until( $node_name );
-					goto in_body_list_done;
+					$this->state->stack_of_open_elements->pop_until( 'FORM' );
+					return true;
 				}
-
-				if (
-					'ADDRESS' !== $node->node_name &&
-					'DIV' !== $node->node_name &&
-					'P' !== $node->node_name &&
-					$this->is_special( $node->node_name )
-				) {
-					/*
-					 * > If node is in the special category, but is not an address, div,
-					 * > or p element, then jump to the step labeled done below.
-					 */
-					goto in_body_list_done;
-				} else {
-					/*
-					 * > Otherwise, set node to the previous entry in the stack of open elements
-					 * > and return to the step labeled loop.
-					 */
-					foreach ( $this->state->stack_of_open_elements->walk_up( $node ) as $item ) {
-						$node = $item;
-						break;
-					}
-					goto in_body_list_loop;
+				break;
+
+			/*
+			 * > An end tag whose tag name is "p"
+			 */
+			case '-P':
+				if ( ! $this->state->stack_of_open_elements->has_p_in_button_scope() ) {
+					$this->insert_html_element( $this->state->current_token );
 				}
 
-				in_body_list_done:
-				if ( $this->state->stack_of_open_elements->has_p_in_button_scope() ) {
-					$this->close_a_p_element();
-				}
-
-				$this->insert_html_element( $this->state->current_token );
+				$this->close_a_p_element();
 				return true;
 
 			/*
@@ -1197,7 +2691,7 @@
 
 				$this->generate_implied_end_tags( $token_name );
 
-				if ( $token_name !== $this->state->stack_of_open_elements->current_node()->node_name ) {
+				if ( ! $this->state->stack_of_open_elements->current_node_is( $token_name ) ) {
 					// @todo Indicate a parse error once it's possible. This error does not impact the logic here.
 				}
 
@@ -1205,28 +2699,46 @@
 				return true;
 
 			/*
-			 * > An end tag whose tag name is "p"
-			 */
-			case '-P':
-				if ( ! $this->state->stack_of_open_elements->has_p_in_button_scope() ) {
-					$this->insert_html_element( $this->state->current_token );
+			 * > An end tag whose tag name is one of: "h1", "h2", "h3", "h4", "h5", "h6"
+			 */
+			case '-H1':
+			case '-H2':
+			case '-H3':
+			case '-H4':
+			case '-H5':
+			case '-H6':
+				if ( ! $this->state->stack_of_open_elements->has_element_in_scope( '(internal: H1 through H6 - do not use)' ) ) {
+					/*
+					 * This is a parse error; ignore the token.
+					 *
+					 * @todo Indicate a parse error once it's possible.
+					 */
+					return $this->step();
 				}
 
-				$this->close_a_p_element();
+				$this->generate_implied_end_tags();
+
+				if ( ! $this->state->stack_of_open_elements->current_node_is( $token_name ) ) {
+					// @todo Record parse error: this error doesn't impact parsing.
+				}
+
+				$this->state->stack_of_open_elements->pop_until( '(internal: H1 through H6 - do not use)' );
 				return true;
 
-			// > A start tag whose tag name is "a"
+			/*
+			 * > A start tag whose tag name is "a"
+			 */
 			case '+A':
 				foreach ( $this->state->active_formatting_elements->walk_up() as $item ) {
 					switch ( $item->node_name ) {
 						case 'marker':
-							break;
+							break 2;
 
 						case 'A':
 							$this->run_adoption_agency_algorithm();
 							$this->state->active_formatting_elements->remove_node( $item );
 							$this->state->stack_of_open_elements->remove_node( $item );
-							break;
+							break 2;
 					}
 				}
 
@@ -1257,6 +2769,22 @@
 				return true;
 
 			/*
+			 * > A start tag whose tag name is "nobr"
+			 */
+			case '+NOBR':
+				$this->reconstruct_active_formatting_elements();
+
+				if ( $this->state->stack_of_open_elements->has_element_in_scope( 'NOBR' ) ) {
+					// Parse error.
+					$this->run_adoption_agency_algorithm();
+					$this->reconstruct_active_formatting_elements();
+				}
+
+				$this->insert_html_element( $this->state->current_token );
+				$this->state->active_formatting_elements->push( $this->state->current_token );
+				return true;
+
+			/*
 			 * > An end tag whose tag name is one of: "a", "b", "big", "code", "em", "font", "i",
 			 * > "nobr", "s", "small", "strike", "strong", "tt", "u"
 			 */
@@ -1267,6 +2795,7 @@
 			case '-EM':
 			case '-FONT':
 			case '-I':
+			case '-NOBR':
 			case '-S':
 			case '-SMALL':
 			case '-STRIKE':
@@ -1277,14 +2806,63 @@
 				return true;
 
 			/*
+			 * > A start tag whose tag name is one of: "applet", "marquee", "object"
+			 */
+			case '+APPLET':
+			case '+MARQUEE':
+			case '+OBJECT':
+				$this->reconstruct_active_formatting_elements();
+				$this->insert_html_element( $this->state->current_token );
+				$this->state->active_formatting_elements->insert_marker();
+				$this->state->frameset_ok = false;
+				return true;
+
+			/*
+			 * > A end tag token whose tag name is one of: "applet", "marquee", "object"
+			 */
+			case '-APPLET':
+			case '-MARQUEE':
+			case '-OBJECT':
+				if ( ! $this->state->stack_of_open_elements->has_element_in_scope( $token_name ) ) {
+					// Parse error: ignore the token.
+					return $this->step();
+				}
+
+				$this->generate_implied_end_tags();
+				if ( ! $this->state->stack_of_open_elements->current_node_is( $token_name ) ) {
+					// This is a parse error.
+				}
+
+				$this->state->stack_of_open_elements->pop_until( $token_name );
+				$this->state->active_formatting_elements->clear_up_to_last_marker();
+				return true;
+
+			/*
+			 * > A start tag whose tag name is "table"
+			 */
+			case '+TABLE':
+				/*
+				 * > If the Document is not set to quirks mode, and the stack of open elements
+				 * > has a p element in button scope, then close a p element.
+				 */
+				if (
+					WP_HTML_Tag_Processor::QUIRKS_MODE !== $this->compat_mode &&
+					$this->state->stack_of_open_elements->has_p_in_button_scope()
+				) {
+					$this->close_a_p_element();
+				}
+
+				$this->insert_html_element( $this->state->current_token );
+				$this->state->frameset_ok    = false;
+				$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_TABLE;
+				return true;
+
+			/*
 			 * > An end tag whose tag name is "br"
-			 * >   Parse error. Drop the attributes from the token, and act as described in the next
-			 * >   entry; i.e. act as if this was a "br" start tag token with no attributes, rather
-			 * >   than the end tag token that it actually is.
-			 */
-			case '-BR':
-				$this->last_error = self::ERROR_UNSUPPORTED;
-				throw new WP_HTML_Unsupported_Exception( 'Closing BR tags require unimplemented special handling.' );
+			 *
+			 * This is prevented from happening because the Tag Processor
+			 * reports all closing BR tags as if they were opening tags.
+			 */
 
 			/*
 			 * > A start tag whose tag name is one of: "area", "br", "embed", "img", "keygen", "wbr"
@@ -1306,15 +2884,26 @@
 			case '+INPUT':
 				$this->reconstruct_active_formatting_elements();
 				$this->insert_html_element( $this->state->current_token );
-				$type_attribute = $this->get_attribute( 'type' );
+
 				/*
 				 * > If the token does not have an attribute with the name "type", or if it does,
 				 * > but that attribute's value is not an ASCII case-insensitive match for the
 				 * > string "hidden", then: set the frameset-ok flag to "not ok".
 				 */
+				$type_attribute = $this->get_attribute( 'type' );
 				if ( ! is_string( $type_attribute ) || 'hidden' !== strtolower( $type_attribute ) ) {
 					$this->state->frameset_ok = false;
 				}
+
+				return true;
+
+			/*
+			 * > A start tag whose tag name is one of: "param", "source", "track"
+			 */
+			case '+PARAM':
+			case '+SOURCE':
+			case '+TRACK':
+				$this->insert_html_element( $this->state->current_token );
 				return true;
 
 			/*
@@ -1329,80 +2918,213 @@
 				return true;
 
 			/*
-			 * > A start tag whose tag name is one of: "param", "source", "track"
-			 */
-			case '+PARAM':
-			case '+SOURCE':
-			case '+TRACK':
+			 * > A start tag whose tag name is "image"
+			 */
+			case '+IMAGE':
+				/*
+				 * > Parse error. Change the token's tag name to "img" and reprocess it. (Don't ask.)
+				 *
+				 * Note that this is handled elsewhere, so it should not be possible to reach this code.
+				 */
+				$this->bail( "Cannot process an IMAGE tag. (Don't ask.)" );
+				break;
+
+			/*
+			 * > A start tag whose tag name is "textarea"
+			 */
+			case '+TEXTAREA':
+				$this->insert_html_element( $this->state->current_token );
+
+				/*
+				 * > If the next token is a U+000A LINE FEED (LF) character token, then ignore
+				 * > that token and move on to the next one. (Newlines at the start of
+				 * > textarea elements are ignored as an authoring convenience.)
+				 *
+				 * This is handled in `get_modifiable_text()`.
+				 */
+
+				$this->state->frameset_ok = false;
+
+				/*
+				 * > Switch the insertion mode to "text".
+				 *
+				 * As a self-contained node, this behavior is handled in the Tag Processor.
+				 */
+				return true;
+
+			/*
+			 * > A start tag whose tag name is "xmp"
+			 */
+			case '+XMP':
+				if ( $this->state->stack_of_open_elements->has_p_in_button_scope() ) {
+					$this->close_a_p_element();
+				}
+
+				$this->reconstruct_active_formatting_elements();
+				$this->state->frameset_ok = false;
+
+				/*
+				 * > Follow the generic raw text element parsing algorithm.
+				 *
+				 * As a self-contained node, this behavior is handled in the Tag Processor.
+				 */
+				$this->insert_html_element( $this->state->current_token );
+				return true;
+
+			/*
+			 * A start tag whose tag name is "iframe"
+			 */
+			case '+IFRAME':
+				$this->state->frameset_ok = false;
+
+				/*
+				 * > Follow the generic raw text element parsing algorithm.
+				 *
+				 * As a self-contained node, this behavior is handled in the Tag Processor.
+				 */
+				$this->insert_html_element( $this->state->current_token );
+				return true;
+
+			/*
+			 * > A start tag whose tag name is "noembed"
+			 * > A start tag whose tag name is "noscript", if the scripting flag is enabled
+			 *
+			 * The scripting flag is never enabled in this parser.
+			 */
+			case '+NOEMBED':
 				$this->insert_html_element( $this->state->current_token );
 				return true;
-		}
-
-		/*
-		 * These tags require special handling in the 'in body' insertion mode
-		 * but that handling hasn't yet been implemented.
-		 *
-		 * As the rules for each tag are implemented, the corresponding tag
-		 * name should be removed from this list. An accompanying test should
-		 * help ensure this list is maintained.
-		 *
-		 * @see Tests_HtmlApi_WpHtmlProcessor::test_step_in_body_fails_on_unsupported_tags
-		 *
-		 * Since this switch structure throws a WP_HTML_Unsupported_Exception, it's
-		 * possible to handle "any other start tag" and "any other end tag" below,
-		 * as that guarantees execution doesn't proceed for the unimplemented tags.
-		 *
-		 * @see https://html.spec.whatwg.org/multipage/parsing.html#parsing-main-inbody
-		 */
-		switch ( $token_name ) {
-			case 'APPLET':
-			case 'BASE':
-			case 'BASEFONT':
-			case 'BGSOUND':
-			case 'BODY':
-			case 'CAPTION':
-			case 'COL':
-			case 'COLGROUP':
-			case 'FORM':
-			case 'FRAME':
-			case 'FRAMESET':
-			case 'HEAD':
-			case 'HTML':
-			case 'IFRAME':
-			case 'LINK':
-			case 'MARQUEE':
-			case 'MATH':
-			case 'META':
-			case 'NOBR':
-			case 'NOEMBED':
-			case 'NOFRAMES':
-			case 'NOSCRIPT':
-			case 'OBJECT':
-			case 'OPTGROUP':
-			case 'OPTION':
-			case 'PLAINTEXT':
-			case 'RB':
-			case 'RP':
-			case 'RT':
-			case 'RTC':
-			case 'SARCASM':
-			case 'SCRIPT':
-			case 'SELECT':
-			case 'STYLE':
-			case 'SVG':
-			case 'TABLE':
-			case 'TBODY':
-			case 'TD':
-			case 'TEMPLATE':
-			case 'TEXTAREA':
-			case 'TFOOT':
-			case 'TH':
-			case 'THEAD':
-			case 'TITLE':
-			case 'TR':
-			case 'XMP':
-				$this->last_error = self::ERROR_UNSUPPORTED;
-				throw new WP_HTML_Unsupported_Exception( "Cannot process {$token_name} element." );
+
+			/*
+			 * > A start tag whose tag name is "select"
+			 */
+			case '+SELECT':
+				$this->reconstruct_active_formatting_elements();
+				$this->insert_html_element( $this->state->current_token );
+				$this->state->frameset_ok = false;
+
+				switch ( $this->state->insertion_mode ) {
+					/*
+					 * > If the insertion mode is one of "in table", "in caption", "in table body", "in row",
+					 * > or "in cell", then switch the insertion mode to "in select in table".
+					 */
+					case WP_HTML_Processor_State::INSERTION_MODE_IN_TABLE:
+					case WP_HTML_Processor_State::INSERTION_MODE_IN_CAPTION:
+					case WP_HTML_Processor_State::INSERTION_MODE_IN_TABLE_BODY:
+					case WP_HTML_Processor_State::INSERTION_MODE_IN_ROW:
+					case WP_HTML_Processor_State::INSERTION_MODE_IN_CELL:
+						$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_SELECT_IN_TABLE;
+						break;
+
+					/*
+					 * > Otherwise, switch the insertion mode to "in select".
+					 */
+					default:
+						$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_SELECT;
+						break;
+				}
+				return true;
+
+			/*
+			 * > A start tag whose tag name is one of: "optgroup", "option"
+			 */
+			case '+OPTGROUP':
+			case '+OPTION':
+				if ( $this->state->stack_of_open_elements->current_node_is( 'OPTION' ) ) {
+					$this->state->stack_of_open_elements->pop();
+				}
+				$this->reconstruct_active_formatting_elements();
+				$this->insert_html_element( $this->state->current_token );
+				return true;
+
+			/*
+			 * > A start tag whose tag name is one of: "rb", "rtc"
+			 */
+			case '+RB':
+			case '+RTC':
+				if ( $this->state->stack_of_open_elements->has_element_in_scope( 'RUBY' ) ) {
+					$this->generate_implied_end_tags();
+
+					if ( $this->state->stack_of_open_elements->current_node_is( 'RUBY' ) ) {
+						// @todo Indicate a parse error once it's possible.
+					}
+				}
+
+				$this->insert_html_element( $this->state->current_token );
+				return true;
+
+			/*
+			 * > A start tag whose tag name is one of: "rp", "rt"
+			 */
+			case '+RP':
+			case '+RT':
+				if ( $this->state->stack_of_open_elements->has_element_in_scope( 'RUBY' ) ) {
+					$this->generate_implied_end_tags( 'RTC' );
+
+					$current_node_name = $this->state->stack_of_open_elements->current_node()->node_name;
+					if ( 'RTC' === $current_node_name || 'RUBY' === $current_node_name ) {
+						// @todo Indicate a parse error once it's possible.
+					}
+				}
+
+				$this->insert_html_element( $this->state->current_token );
+				return true;
+
+			/*
+			 * > A start tag whose tag name is "math"
+			 */
+			case '+MATH':
+				$this->reconstruct_active_formatting_elements();
+
+				/*
+				 * @todo Adjust MathML attributes for the token. (This fixes the case of MathML attributes that are not all lowercase.)
+				 * @todo Adjust foreign attributes for the token. (This fixes the use of namespaced attributes, in particular XLink.)
+				 *
+				 * These ought to be handled in the attribute methods.
+				 */
+				$this->state->current_token->namespace = 'math';
+				$this->insert_html_element( $this->state->current_token );
+				if ( $this->state->current_token->has_self_closing_flag ) {
+					$this->state->stack_of_open_elements->pop();
+				}
+				return true;
+
+			/*
+			 * > A start tag whose tag name is "svg"
+			 */
+			case '+SVG':
+				$this->reconstruct_active_formatting_elements();
+
+				/*
+				 * @todo Adjust SVG attributes for the token. (This fixes the case of SVG attributes that are not all lowercase.)
+				 * @todo Adjust foreign attributes for the token. (This fixes the use of namespaced attributes, in particular XLink in SVG.)
+				 *
+				 * These ought to be handled in the attribute methods.
+				 */
+				$this->state->current_token->namespace = 'svg';
+				$this->insert_html_element( $this->state->current_token );
+				if ( $this->state->current_token->has_self_closing_flag ) {
+					$this->state->stack_of_open_elements->pop();
+				}
+				return true;
+
+			/*
+			 * > A start tag whose tag name is one of: "caption", "col", "colgroup",
+			 * > "frame", "head", "tbody", "td", "tfoot", "th", "thead", "tr"
+			 */
+			case '+CAPTION':
+			case '+COL':
+			case '+COLGROUP':
+			case '+FRAME':
+			case '+HEAD':
+			case '+TBODY':
+			case '+TD':
+			case '+TFOOT':
+			case '+TH':
+			case '+THEAD':
+			case '+TR':
+				// Parse error. Ignore the token.
+				return $this->step();
 		}
 
 		if ( ! parent::is_tag_closer() ) {
@@ -1424,11 +3146,11 @@
 			 * close anything beyond its containing `P` or `DIV` element.
 			 */
 			foreach ( $this->state->stack_of_open_elements->walk_up() as $node ) {
-				if ( $token_name === $node->node_name ) {
+				if ( 'html' === $node->namespace && $token_name === $node->node_name ) {
 					break;
 				}
 
-				if ( self::is_special( $node->node_name ) ) {
+				if ( self::is_special( $node ) ) {
 					// This is a parse error, ignore the token.
 					return $this->step();
 				}
@@ -1446,6 +3168,1904 @@
 				}
 			}
 		}
+
+		$this->bail( 'Should not have been able to reach end of IN BODY processing. Check HTML API code.' );
+		// This unnecessary return prevents tools from inaccurately reporting type errors.
+		return false;
+	}
+
+	/**
+	 * Parses next element in the 'in table' insertion mode.
+	 *
+	 * This internal function performs the 'in table' insertion mode
+	 * logic for the generalized WP_HTML_Processor::step() function.
+	 *
+	 * @since 6.7.0
+	 *
+	 * @throws WP_HTML_Unsupported_Exception When encountering unsupported HTML input.
+	 *
+	 * @see https://html.spec.whatwg.org/#parsing-main-intable
+	 * @see WP_HTML_Processor::step
+	 *
+	 * @return bool Whether an element was found.
+	 */
+	private function step_in_table(): bool {
+		$token_name = $this->get_token_name();
+		$token_type = $this->get_token_type();
+		$op_sigil   = '#tag' === $token_type ? ( parent::is_tag_closer() ? '-' : '+' ) : '';
+		$op         = "{$op_sigil}{$token_name}";
+
+		switch ( $op ) {
+			/*
+			 * > A character token, if the current node is table,
+			 * > tbody, template, tfoot, thead, or tr element
+			 */
+			case '#text':
+				$current_node      = $this->state->stack_of_open_elements->current_node();
+				$current_node_name = $current_node ? $current_node->node_name : null;
+				if (
+					$current_node_name && (
+						'TABLE' === $current_node_name ||
+						'TBODY' === $current_node_name ||
+						'TEMPLATE' === $current_node_name ||
+						'TFOOT' === $current_node_name ||
+						'THEAD' === $current_node_name ||
+						'TR' === $current_node_name
+					)
+				) {
+					/*
+					 * If the text is empty after processing HTML entities and stripping
+					 * U+0000 NULL bytes then ignore the token.
+					 */
+					if ( parent::TEXT_IS_NULL_SEQUENCE === $this->text_node_classification ) {
+						return $this->step();
+					}
+
+					/*
+					 * This follows the rules for "in table text" insertion mode.
+					 *
+					 * Whitespace-only text nodes are inserted in-place. Otherwise
+					 * foster parenting is enabled and the nodes would be
+					 * inserted out-of-place.
+					 *
+					 * > If any of the tokens in the pending table character tokens
+					 * > list are character tokens that are not ASCII whitespace,
+					 * > then this is a parse error: reprocess the character tokens
+					 * > in the pending table character tokens list using the rules
+					 * > given in the "anything else" entry in the "in table"
+					 * > insertion mode.
+					 * >
+					 * > Otherwise, insert the characters given by the pending table
+					 * > character tokens list.
+					 *
+					 * @see https://html.spec.whatwg.org/#parsing-main-intabletext
+					 */
+					if ( parent::TEXT_IS_WHITESPACE === $this->text_node_classification ) {
+						$this->insert_html_element( $this->state->current_token );
+						return true;
+					}
+
+					// Non-whitespace would trigger fostering, unsupported at this time.
+					$this->bail( 'Foster parenting is not supported.' );
+					break;
+				}
+				break;
+
+			/*
+			 * > A comment token
+			 */
+			case '#comment':
+			case '#funky-comment':
+			case '#presumptuous-tag':
+				$this->insert_html_element( $this->state->current_token );
+				return true;
+
+			/*
+			 * > A DOCTYPE token
+			 */
+			case 'html':
+				// Parse error: ignore the token.
+				return $this->step();
+
+			/*
+			 * > A start tag whose tag name is "caption"
+			 */
+			case '+CAPTION':
+				$this->state->stack_of_open_elements->clear_to_table_context();
+				$this->state->active_formatting_elements->insert_marker();
+				$this->insert_html_element( $this->state->current_token );
+				$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_CAPTION;
+				return true;
+
+			/*
+			 * > A start tag whose tag name is "colgroup"
+			 */
+			case '+COLGROUP':
+				$this->state->stack_of_open_elements->clear_to_table_context();
+				$this->insert_html_element( $this->state->current_token );
+				$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_COLUMN_GROUP;
+				return true;
+
+			/*
+			 * > A start tag whose tag name is "col"
+			 */
+			case '+COL':
+				$this->state->stack_of_open_elements->clear_to_table_context();
+
+				/*
+				 * > Insert an HTML element for a "colgroup" start tag token with no attributes,
+				 * > then switch the insertion mode to "in column group".
+				 */
+				$this->insert_virtual_node( 'COLGROUP' );
+				$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_COLUMN_GROUP;
+				return $this->step( self::REPROCESS_CURRENT_NODE );
+
+			/*
+			 * > A start tag whose tag name is one of: "tbody", "tfoot", "thead"
+			 */
+			case '+TBODY':
+			case '+TFOOT':
+			case '+THEAD':
+				$this->state->stack_of_open_elements->clear_to_table_context();
+				$this->insert_html_element( $this->state->current_token );
+				$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_TABLE_BODY;
+				return true;
+
+			/*
+			 * > A start tag whose tag name is one of: "td", "th", "tr"
+			 */
+			case '+TD':
+			case '+TH':
+			case '+TR':
+				$this->state->stack_of_open_elements->clear_to_table_context();
+				/*
+				 * > Insert an HTML element for a "tbody" start tag token with no attributes,
+				 * > then switch the insertion mode to "in table body".
+				 */
+				$this->insert_virtual_node( 'TBODY' );
+				$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_TABLE_BODY;
+				return $this->step( self::REPROCESS_CURRENT_NODE );
+
+			/*
+			 * > A start tag whose tag name is "table"
+			 *
+			 * This tag in the IN TABLE insertion mode is a parse error.
+			 */
+			case '+TABLE':
+				if ( ! $this->state->stack_of_open_elements->has_element_in_table_scope( 'TABLE' ) ) {
+					return $this->step();
+				}
+
+				$this->state->stack_of_open_elements->pop_until( 'TABLE' );
+				$this->reset_insertion_mode_appropriately();
+				return $this->step( self::REPROCESS_CURRENT_NODE );
+
+			/*
+			 * > An end tag whose tag name is "table"
+			 */
+			case '-TABLE':
+				if ( ! $this->state->stack_of_open_elements->has_element_in_table_scope( 'TABLE' ) ) {
+					// @todo Indicate a parse error once it's possible.
+					return $this->step();
+				}
+
+				$this->state->stack_of_open_elements->pop_until( 'TABLE' );
+				$this->reset_insertion_mode_appropriately();
+				return true;
+
+			/*
+			 * > An end tag whose tag name is one of: "body", "caption", "col", "colgroup", "html", "tbody", "td", "tfoot", "th", "thead", "tr"
+			 */
+			case '-BODY':
+			case '-CAPTION':
+			case '-COL':
+			case '-COLGROUP':
+			case '-HTML':
+			case '-TBODY':
+			case '-TD':
+			case '-TFOOT':
+			case '-TH':
+			case '-THEAD':
+			case '-TR':
+				// Parse error: ignore the token.
+				return $this->step();
+
+			/*
+			 * > A start tag whose tag name is one of: "style", "script", "template"
+			 * > An end tag whose tag name is "template"
+			 */
+			case '+STYLE':
+			case '+SCRIPT':
+			case '+TEMPLATE':
+			case '-TEMPLATE':
+				/*
+				 * > Process the token using the rules for the "in head" insertion mode.
+				 */
+				return $this->step_in_head();
+
+			/*
+			 * > A start tag whose tag name is "input"
+			 *
+			 * > If the token does not have an attribute with the name "type", or if it does, but
+			 * > that attribute's value is not an ASCII case-insensitive match for the string
+			 * > "hidden", then: act as described in the "anything else" entry below.
+			 */
+			case '+INPUT':
+				$type_attribute = $this->get_attribute( 'type' );
+				if ( ! is_string( $type_attribute ) || 'hidden' !== strtolower( $type_attribute ) ) {
+					goto anything_else;
+				}
+				// @todo Indicate a parse error once it's possible.
+				$this->insert_html_element( $this->state->current_token );
+				return true;
+
+			/*
+			 * > A start tag whose tag name is "form"
+			 *
+			 * This tag in the IN TABLE insertion mode is a parse error.
+			 */
+			case '+FORM':
+				if (
+					$this->state->stack_of_open_elements->has_element_in_scope( 'TEMPLATE' ) ||
+					isset( $this->state->form_element )
+				) {
+					return $this->step();
+				}
+
+				// This FORM is special because it immediately closes and cannot have other children.
+				$this->insert_html_element( $this->state->current_token );
+				$this->state->form_element = $this->state->current_token;
+				$this->state->stack_of_open_elements->pop();
+				return true;
+		}
+
+		/*
+		 * > Anything else
+		 * > Parse error. Enable foster parenting, process the token using the rules for the
+		 * > "in body" insertion mode, and then disable foster parenting.
+		 *
+		 * @todo Indicate a parse error once it's possible.
+		 */
+		anything_else:
+		$this->bail( 'Foster parenting is not supported.' );
+	}
+
+	/**
+	 * Parses next element in the 'in table text' insertion mode.
+	 *
+	 * This internal function performs the 'in table text' insertion mode
+	 * logic for the generalized WP_HTML_Processor::step() function.
+	 *
+	 * @since 6.7.0 Stub implementation.
+	 *
+	 * @throws WP_HTML_Unsupported_Exception When encountering unsupported HTML input.
+	 *
+	 * @see https://html.spec.whatwg.org/#parsing-main-intabletext
+	 * @see WP_HTML_Processor::step
+	 *
+	 * @return bool Whether an element was found.
+	 */
+	private function step_in_table_text(): bool {
+		$this->bail( 'No support for parsing in the ' . WP_HTML_Processor_State::INSERTION_MODE_IN_TABLE_TEXT . ' state.' );
+	}
+
+	/**
+	 * Parses next element in the 'in caption' insertion mode.
+	 *
+	 * This internal function performs the 'in caption' insertion mode
+	 * logic for the generalized WP_HTML_Processor::step() function.
+	 *
+	 * @since 6.7.0
+	 *
+	 * @throws WP_HTML_Unsupported_Exception When encountering unsupported HTML input.
+	 *
+	 * @see https://html.spec.whatwg.org/#parsing-main-incaption
+	 * @see WP_HTML_Processor::step
+	 *
+	 * @return bool Whether an element was found.
+	 */
+	private function step_in_caption(): bool {
+		$tag_name = $this->get_tag();
+		$op_sigil = $this->is_tag_closer() ? '-' : '+';
+		$op       = "{$op_sigil}{$tag_name}";
+
+		switch ( $op ) {
+			/*
+			 * > An end tag whose tag name is "caption"
+			 * > A start tag whose tag name is one of: "caption", "col", "colgroup", "tbody", "td", "tfoot", "th", "thead", "tr"
+			 * > An end tag whose tag name is "table"
+			 *
+			 * These tag handling rules are identical except for the final instruction.
+			 * Handle them in a single block.
+			 */
+			case '-CAPTION':
+			case '+CAPTION':
+			case '+COL':
+			case '+COLGROUP':
+			case '+TBODY':
+			case '+TD':
+			case '+TFOOT':
+			case '+TH':
+			case '+THEAD':
+			case '+TR':
+			case '-TABLE':
+				if ( ! $this->state->stack_of_open_elements->has_element_in_table_scope( 'CAPTION' ) ) {
+					// Parse error: ignore the token.
+					return $this->step();
+				}
+
+				$this->generate_implied_end_tags();
+				if ( ! $this->state->stack_of_open_elements->current_node_is( 'CAPTION' ) ) {
+					// @todo Indicate a parse error once it's possible.
+				}
+
+				$this->state->stack_of_open_elements->pop_until( 'CAPTION' );
+				$this->state->active_formatting_elements->clear_up_to_last_marker();
+				$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_TABLE;
+
+				// If this is not a CAPTION end tag, the token should be reprocessed.
+				if ( '-CAPTION' === $op ) {
+					return true;
+				}
+				return $this->step( self::REPROCESS_CURRENT_NODE );
+
+			/**
+			 * > An end tag whose tag name is one of: "body", "col", "colgroup", "html", "tbody", "td", "tfoot", "th", "thead", "tr"
+			 */
+			case '-BODY':
+			case '-COL':
+			case '-COLGROUP':
+			case '-HTML':
+			case '-TBODY':
+			case '-TD':
+			case '-TFOOT':
+			case '-TH':
+			case '-THEAD':
+			case '-TR':
+				// Parse error: ignore the token.
+				return $this->step();
+		}
+
+		/**
+		 * > Anything else
+		 * >   Process the token using the rules for the "in body" insertion mode.
+		 */
+		return $this->step_in_body();
+	}
+
+	/**
+	 * Parses next element in the 'in column group' insertion mode.
+	 *
+	 * This internal function performs the 'in column group' insertion mode
+	 * logic for the generalized WP_HTML_Processor::step() function.
+	 *
+	 * @since 6.7.0
+	 *
+	 * @throws WP_HTML_Unsupported_Exception When encountering unsupported HTML input.
+	 *
+	 * @see https://html.spec.whatwg.org/#parsing-main-incolgroup
+	 * @see WP_HTML_Processor::step
+	 *
+	 * @return bool Whether an element was found.
+	 */
+	private function step_in_column_group(): bool {
+		$token_name = $this->get_token_name();
+		$token_type = $this->get_token_type();
+		$op_sigil   = '#tag' === $token_type ? ( parent::is_tag_closer() ? '-' : '+' ) : '';
+		$op         = "{$op_sigil}{$token_name}";
+
+		switch ( $op ) {
+			/*
+			 * > A character token that is one of U+0009 CHARACTER TABULATION, U+000A LINE FEED (LF),
+			 * > U+000C FORM FEED (FF), U+000D CARRIAGE RETURN (CR), or U+0020 SPACE
+			 */
+			case '#text':
+				if ( parent::TEXT_IS_WHITESPACE === $this->text_node_classification ) {
+					// Insert the character.
+					$this->insert_html_element( $this->state->current_token );
+					return true;
+				}
+
+				goto in_column_group_anything_else;
+				break;
+
+			/*
+			 * > A comment token
+			 */
+			case '#comment':
+			case '#funky-comment':
+			case '#presumptuous-tag':
+				$this->insert_html_element( $this->state->current_token );
+				return true;
+
+			/*
+			 * > A DOCTYPE token
+			 */
+			case 'html':
+				// @todo Indicate a parse error once it's possible.
+				return $this->step();
+
+			/*
+			 * > A start tag whose tag name is "html"
+			 */
+			case '+HTML':
+				return $this->step_in_body();
+
+			/*
+			 * > A start tag whose tag name is "col"
+			 */
+			case '+COL':
+				$this->insert_html_element( $this->state->current_token );
+				$this->state->stack_of_open_elements->pop();
+				return true;
+
+			/*
+			 * > An end tag whose tag name is "colgroup"
+			 */
+			case '-COLGROUP':
+				if ( ! $this->state->stack_of_open_elements->current_node_is( 'COLGROUP' ) ) {
+					// @todo Indicate a parse error once it's possible.
+					return $this->step();
+				}
+				$this->state->stack_of_open_elements->pop();
+				$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_TABLE;
+				return true;
+
+			/*
+			 * > An end tag whose tag name is "col"
+			 */
+			case '-COL':
+				// Parse error: ignore the token.
+				return $this->step();
+
+			/*
+			 * > A start tag whose tag name is "template"
+			 * > An end tag whose tag name is "template"
+			 */
+			case '+TEMPLATE':
+			case '-TEMPLATE':
+				return $this->step_in_head();
+		}
+
+		in_column_group_anything_else:
+		/*
+		 * > Anything else
+		 */
+		if ( ! $this->state->stack_of_open_elements->current_node_is( 'COLGROUP' ) ) {
+			// @todo Indicate a parse error once it's possible.
+			return $this->step();
+		}
+		$this->state->stack_of_open_elements->pop();
+		$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_TABLE;
+		return $this->step( self::REPROCESS_CURRENT_NODE );
+	}
+
+	/**
+	 * Parses next element in the 'in table body' insertion mode.
+	 *
+	 * This internal function performs the 'in table body' insertion mode
+	 * logic for the generalized WP_HTML_Processor::step() function.
+	 *
+	 * @since 6.7.0
+	 *
+	 * @throws WP_HTML_Unsupported_Exception When encountering unsupported HTML input.
+	 *
+	 * @see https://html.spec.whatwg.org/#parsing-main-intbody
+	 * @see WP_HTML_Processor::step
+	 *
+	 * @return bool Whether an element was found.
+	 */
+	private function step_in_table_body(): bool {
+		$tag_name = $this->get_tag();
+		$op_sigil = $this->is_tag_closer() ? '-' : '+';
+		$op       = "{$op_sigil}{$tag_name}";
+
+		switch ( $op ) {
+			/*
+			 * > A start tag whose tag name is "tr"
+			 */
+			case '+TR':
+				$this->state->stack_of_open_elements->clear_to_table_body_context();
+				$this->insert_html_element( $this->state->current_token );
+				$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_ROW;
+				return true;
+
+			/*
+			 * > A start tag whose tag name is one of: "th", "td"
+			 */
+			case '+TH':
+			case '+TD':
+				// @todo Indicate a parse error once it's possible.
+				$this->state->stack_of_open_elements->clear_to_table_body_context();
+				$this->insert_virtual_node( 'TR' );
+				$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_ROW;
+				return $this->step( self::REPROCESS_CURRENT_NODE );
+
+			/*
+			 * > An end tag whose tag name is one of: "tbody", "tfoot", "thead"
+			 */
+			case '-TBODY':
+			case '-TFOOT':
+			case '-THEAD':
+				if ( ! $this->state->stack_of_open_elements->has_element_in_table_scope( $tag_name ) ) {
+					// Parse error: ignore the token.
+					return $this->step();
+				}
+
+				$this->state->stack_of_open_elements->clear_to_table_body_context();
+				$this->state->stack_of_open_elements->pop();
+				$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_TABLE;
+				return true;
+
+			/*
+			 * > A start tag whose tag name is one of: "caption", "col", "colgroup", "tbody", "tfoot", "thead"
+			 * > An end tag whose tag name is "table"
+			 */
+			case '+CAPTION':
+			case '+COL':
+			case '+COLGROUP':
+			case '+TBODY':
+			case '+TFOOT':
+			case '+THEAD':
+			case '-TABLE':
+				if (
+					! $this->state->stack_of_open_elements->has_element_in_table_scope( 'TBODY' ) &&
+					! $this->state->stack_of_open_elements->has_element_in_table_scope( 'THEAD' ) &&
+					! $this->state->stack_of_open_elements->has_element_in_table_scope( 'TFOOT' )
+				) {
+					// Parse error: ignore the token.
+					return $this->step();
+				}
+				$this->state->stack_of_open_elements->clear_to_table_body_context();
+				$this->state->stack_of_open_elements->pop();
+				$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_TABLE;
+				return $this->step( self::REPROCESS_CURRENT_NODE );
+
+			/*
+			 * > An end tag whose tag name is one of: "body", "caption", "col", "colgroup", "html", "td", "th", "tr"
+			 */
+			case '-BODY':
+			case '-CAPTION':
+			case '-COL':
+			case '-COLGROUP':
+			case '-HTML':
+			case '-TD':
+			case '-TH':
+			case '-TR':
+				// Parse error: ignore the token.
+				return $this->step();
+		}
+
+		/*
+		 * > Anything else
+		 * > Process the token using the rules for the "in table" insertion mode.
+		 */
+		return $this->step_in_table();
+	}
+
+	/**
+	 * Parses next element in the 'in row' insertion mode.
+	 *
+	 * This internal function performs the 'in row' insertion mode
+	 * logic for the generalized WP_HTML_Processor::step() function.
+	 *
+	 * @since 6.7.0
+	 *
+	 * @throws WP_HTML_Unsupported_Exception When encountering unsupported HTML input.
+	 *
+	 * @see https://html.spec.whatwg.org/#parsing-main-intr
+	 * @see WP_HTML_Processor::step
+	 *
+	 * @return bool Whether an element was found.
+	 */
+	private function step_in_row(): bool {
+		$tag_name = $this->get_tag();
+		$op_sigil = $this->is_tag_closer() ? '-' : '+';
+		$op       = "{$op_sigil}{$tag_name}";
+
+		switch ( $op ) {
+			/*
+			 * > A start tag whose tag name is one of: "th", "td"
+			 */
+			case '+TH':
+			case '+TD':
+				$this->state->stack_of_open_elements->clear_to_table_row_context();
+				$this->insert_html_element( $this->state->current_token );
+				$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_CELL;
+				$this->state->active_formatting_elements->insert_marker();
+				return true;
+
+			/*
+			 * > An end tag whose tag name is "tr"
+			 */
+			case '-TR':
+				if ( ! $this->state->stack_of_open_elements->has_element_in_table_scope( 'TR' ) ) {
+					// Parse error: ignore the token.
+					return $this->step();
+				}
+
+				$this->state->stack_of_open_elements->clear_to_table_row_context();
+				$this->state->stack_of_open_elements->pop();
+				$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_TABLE_BODY;
+				return true;
+
+			/*
+			 * > A start tag whose tag name is one of: "caption", "col", "colgroup", "tbody", "tfoot", "thead", "tr"
+			 * > An end tag whose tag name is "table"
+			 */
+			case '+CAPTION':
+			case '+COL':
+			case '+COLGROUP':
+			case '+TBODY':
+			case '+TFOOT':
+			case '+THEAD':
+			case '+TR':
+			case '-TABLE':
+				if ( ! $this->state->stack_of_open_elements->has_element_in_table_scope( 'TR' ) ) {
+					// Parse error: ignore the token.
+					return $this->step();
+				}
+
+				$this->state->stack_of_open_elements->clear_to_table_row_context();
+				$this->state->stack_of_open_elements->pop();
+				$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_TABLE_BODY;
+				return $this->step( self::REPROCESS_CURRENT_NODE );
+
+			/*
+			 * > An end tag whose tag name is one of: "tbody", "tfoot", "thead"
+			 */
+			case '-TBODY':
+			case '-TFOOT':
+			case '-THEAD':
+				if ( ! $this->state->stack_of_open_elements->has_element_in_table_scope( $tag_name ) ) {
+					// Parse error: ignore the token.
+					return $this->step();
+				}
+
+				if ( ! $this->state->stack_of_open_elements->has_element_in_table_scope( 'TR' ) ) {
+					// Ignore the token.
+					return $this->step();
+				}
+
+				$this->state->stack_of_open_elements->clear_to_table_row_context();
+				$this->state->stack_of_open_elements->pop();
+				$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_TABLE_BODY;
+				return $this->step( self::REPROCESS_CURRENT_NODE );
+
+			/*
+			 * > An end tag whose tag name is one of: "body", "caption", "col", "colgroup", "html", "td", "th"
+			 */
+			case '-BODY':
+			case '-CAPTION':
+			case '-COL':
+			case '-COLGROUP':
+			case '-HTML':
+			case '-TD':
+			case '-TH':
+				// Parse error: ignore the token.
+				return $this->step();
+		}
+
+		/*
+		 * > Anything else
+		 * >   Process the token using the rules for the "in table" insertion mode.
+		 */
+		return $this->step_in_table();
+	}
+
+	/**
+	 * Parses next element in the 'in cell' insertion mode.
+	 *
+	 * This internal function performs the 'in cell' insertion mode
+	 * logic for the generalized WP_HTML_Processor::step() function.
+	 *
+	 * @since 6.7.0
+	 *
+	 * @throws WP_HTML_Unsupported_Exception When encountering unsupported HTML input.
+	 *
+	 * @see https://html.spec.whatwg.org/#parsing-main-intd
+	 * @see WP_HTML_Processor::step
+	 *
+	 * @return bool Whether an element was found.
+	 */
+	private function step_in_cell(): bool {
+		$tag_name = $this->get_tag();
+		$op_sigil = $this->is_tag_closer() ? '-' : '+';
+		$op       = "{$op_sigil}{$tag_name}";
+
+		switch ( $op ) {
+			/*
+			 * > An end tag whose tag name is one of: "td", "th"
+			 */
+			case '-TD':
+			case '-TH':
+				if ( ! $this->state->stack_of_open_elements->has_element_in_table_scope( $tag_name ) ) {
+					// Parse error: ignore the token.
+					return $this->step();
+				}
+
+				$this->generate_implied_end_tags();
+
+				/*
+				 * @todo This needs to check if the current node is an HTML element, meaning that
+				 *       when SVG and MathML support is added, this needs to differentiate between an
+				 *       HTML element of the given name, such as `<center>`, and a foreign element of
+				 *       the same given name.
+				 */
+				if ( ! $this->state->stack_of_open_elements->current_node_is( $tag_name ) ) {
+					// @todo Indicate a parse error once it's possible.
+				}
+
+				$this->state->stack_of_open_elements->pop_until( $tag_name );
+				$this->state->active_formatting_elements->clear_up_to_last_marker();
+				$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_ROW;
+				return true;
+
+			/*
+			 * > A start tag whose tag name is one of: "caption", "col", "colgroup", "tbody", "td",
+			 * > "tfoot", "th", "thead", "tr"
+			 */
+			case '+CAPTION':
+			case '+COL':
+			case '+COLGROUP':
+			case '+TBODY':
+			case '+TD':
+			case '+TFOOT':
+			case '+TH':
+			case '+THEAD':
+			case '+TR':
+				/*
+				 * > Assert: The stack of open elements has a td or th element in table scope.
+				 *
+				 * Nothing to do here, except to verify in tests that this never appears.
+				 */
+
+				$this->close_cell();
+				return $this->step( self::REPROCESS_CURRENT_NODE );
+
+			/*
+			 * > An end tag whose tag name is one of: "body", "caption", "col", "colgroup", "html"
+			 */
+			case '-BODY':
+			case '-CAPTION':
+			case '-COL':
+			case '-COLGROUP':
+			case '-HTML':
+				// Parse error: ignore the token.
+				return $this->step();
+
+			/*
+			 * > An end tag whose tag name is one of: "table", "tbody", "tfoot", "thead", "tr"
+			 */
+			case '-TABLE':
+			case '-TBODY':
+			case '-TFOOT':
+			case '-THEAD':
+			case '-TR':
+				if ( ! $this->state->stack_of_open_elements->has_element_in_table_scope( $tag_name ) ) {
+					// Parse error: ignore the token.
+					return $this->step();
+				}
+				$this->close_cell();
+				return $this->step( self::REPROCESS_CURRENT_NODE );
+		}
+
+		/*
+		 * > Anything else
+		 * >   Process the token using the rules for the "in body" insertion mode.
+		 */
+		return $this->step_in_body();
+	}
+
+	/**
+	 * Parses next element in the 'in select' insertion mode.
+	 *
+	 * This internal function performs the 'in select' insertion mode
+	 * logic for the generalized WP_HTML_Processor::step() function.
+	 *
+	 * @since 6.7.0
+	 *
+	 * @throws WP_HTML_Unsupported_Exception When encountering unsupported HTML input.
+	 *
+	 * @see https://html.spec.whatwg.org/multipage/parsing.html#parsing-main-inselect
+	 * @see WP_HTML_Processor::step
+	 *
+	 * @return bool Whether an element was found.
+	 */
+	private function step_in_select(): bool {
+		$token_name = $this->get_token_name();
+		$token_type = $this->get_token_type();
+		$op_sigil   = '#tag' === $token_type ? ( parent::is_tag_closer() ? '-' : '+' ) : '';
+		$op         = "{$op_sigil}{$token_name}";
+
+		switch ( $op ) {
+			/*
+			 * > Any other character token
+			 */
+			case '#text':
+				/*
+				 * > A character token that is U+0000 NULL
+				 *
+				 * If a text node only comprises null bytes then it should be
+				 * entirely ignored and should not return to calling code.
+				 */
+				if ( parent::TEXT_IS_NULL_SEQUENCE === $this->text_node_classification ) {
+					// Parse error: ignore the token.
+					return $this->step();
+				}
+
+				$this->insert_html_element( $this->state->current_token );
+				return true;
+
+			/*
+			 * > A comment token
+			 */
+			case '#comment':
+			case '#funky-comment':
+			case '#presumptuous-tag':
+				$this->insert_html_element( $this->state->current_token );
+				return true;
+
+			/*
+			 * > A DOCTYPE token
+			 */
+			case 'html':
+				// Parse error: ignore the token.
+				return $this->step();
+
+			/*
+			 * > A start tag whose tag name is "html"
+			 */
+			case '+HTML':
+				return $this->step_in_body();
+
+			/*
+			 * > A start tag whose tag name is "option"
+			 */
+			case '+OPTION':
+				if ( $this->state->stack_of_open_elements->current_node_is( 'OPTION' ) ) {
+					$this->state->stack_of_open_elements->pop();
+				}
+				$this->insert_html_element( $this->state->current_token );
+				return true;
+
+			/*
+			 * > A start tag whose tag name is "optgroup"
+			 * > A start tag whose tag name is "hr"
+			 *
+			 * These rules are identical except for the treatment of the self-closing flag and
+			 * the subsequent pop of the HR void element, all of which is handled elsewhere in the processor.
+			 */
+			case '+OPTGROUP':
+			case '+HR':
+				if ( $this->state->stack_of_open_elements->current_node_is( 'OPTION' ) ) {
+					$this->state->stack_of_open_elements->pop();
+				}
+
+				if ( $this->state->stack_of_open_elements->current_node_is( 'OPTGROUP' ) ) {
+					$this->state->stack_of_open_elements->pop();
+				}
+
+				$this->insert_html_element( $this->state->current_token );
+				return true;
+
+			/*
+			 * > An end tag whose tag name is "optgroup"
+			 */
+			case '-OPTGROUP':
+				$current_node = $this->state->stack_of_open_elements->current_node();
+				if ( $current_node && 'OPTION' === $current_node->node_name ) {
+					foreach ( $this->state->stack_of_open_elements->walk_up( $current_node ) as $parent ) {
+						break;
+					}
+					if ( $parent && 'OPTGROUP' === $parent->node_name ) {
+						$this->state->stack_of_open_elements->pop();
+					}
+				}
+
+				if ( $this->state->stack_of_open_elements->current_node_is( 'OPTGROUP' ) ) {
+					$this->state->stack_of_open_elements->pop();
+					return true;
+				}
+
+				// Parse error: ignore the token.
+				return $this->step();
+
+			/*
+			 * > An end tag whose tag name is "option"
+			 */
+			case '-OPTION':
+				if ( $this->state->stack_of_open_elements->current_node_is( 'OPTION' ) ) {
+					$this->state->stack_of_open_elements->pop();
+					return true;
+				}
+
+				// Parse error: ignore the token.
+				return $this->step();
+
+			/*
+			 * > An end tag whose tag name is "select"
+			 * > A start tag whose tag name is "select"
+			 *
+			 * > It just gets treated like an end tag.
+			 */
+			case '-SELECT':
+			case '+SELECT':
+				if ( ! $this->state->stack_of_open_elements->has_element_in_select_scope( 'SELECT' ) ) {
+					// Parse error: ignore the token.
+					return $this->step();
+				}
+				$this->state->stack_of_open_elements->pop_until( 'SELECT' );
+				$this->reset_insertion_mode_appropriately();
+				return true;
+
+			/*
+			 * > A start tag whose tag name is one of: "input", "keygen", "textarea"
+			 *
+			 * All three of these tags are considered a parse error when found in this insertion mode.
+			 */
+			case '+INPUT':
+			case '+KEYGEN':
+			case '+TEXTAREA':
+				if ( ! $this->state->stack_of_open_elements->has_element_in_select_scope( 'SELECT' ) ) {
+					// Ignore the token.
+					return $this->step();
+				}
+				$this->state->stack_of_open_elements->pop_until( 'SELECT' );
+				$this->reset_insertion_mode_appropriately();
+				return $this->step( self::REPROCESS_CURRENT_NODE );
+
+			/*
+			 * > A start tag whose tag name is one of: "script", "template"
+			 * > An end tag whose tag name is "template"
+			 */
+			case '+SCRIPT':
+			case '+TEMPLATE':
+			case '-TEMPLATE':
+				return $this->step_in_head();
+		}
+
+		/*
+		 * > Anything else
+		 * >   Parse error: ignore the token.
+		 */
+		return $this->step();
+	}
+
+	/**
+	 * Parses next element in the 'in select in table' insertion mode.
+	 *
+	 * This internal function performs the 'in select in table' insertion mode
+	 * logic for the generalized WP_HTML_Processor::step() function.
+	 *
+	 * @since 6.7.0
+	 *
+	 * @throws WP_HTML_Unsupported_Exception When encountering unsupported HTML input.
+	 *
+	 * @see https://html.spec.whatwg.org/#parsing-main-inselectintable
+	 * @see WP_HTML_Processor::step
+	 *
+	 * @return bool Whether an element was found.
+	 */
+	private function step_in_select_in_table(): bool {
+		$token_name = $this->get_token_name();
+		$token_type = $this->get_token_type();
+		$op_sigil   = '#tag' === $token_type ? ( parent::is_tag_closer() ? '-' : '+' ) : '';
+		$op         = "{$op_sigil}{$token_name}";
+
+		switch ( $op ) {
+			/*
+			 * > A start tag whose tag name is one of: "caption", "table", "tbody", "tfoot", "thead", "tr", "td", "th"
+			 */
+			case '+CAPTION':
+			case '+TABLE':
+			case '+TBODY':
+			case '+TFOOT':
+			case '+THEAD':
+			case '+TR':
+			case '+TD':
+			case '+TH':
+				// @todo Indicate a parse error once it's possible.
+				$this->state->stack_of_open_elements->pop_until( 'SELECT' );
+				$this->reset_insertion_mode_appropriately();
+				return $this->step( self::REPROCESS_CURRENT_NODE );
+
+			/*
+			 * > An end tag whose tag name is one of: "caption", "table", "tbody", "tfoot", "thead", "tr", "td", "th"
+			 */
+			case '-CAPTION':
+			case '-TABLE':
+			case '-TBODY':
+			case '-TFOOT':
+			case '-THEAD':
+			case '-TR':
+			case '-TD':
+			case '-TH':
+				// @todo Indicate a parse error once it's possible.
+				if ( ! $this->state->stack_of_open_elements->has_element_in_table_scope( $token_name ) ) {
+					return $this->step();
+				}
+				$this->state->stack_of_open_elements->pop_until( 'SELECT' );
+				$this->reset_insertion_mode_appropriately();
+				return $this->step( self::REPROCESS_CURRENT_NODE );
+		}
+
+		/*
+		 * > Anything else
+		 */
+		return $this->step_in_select();
+	}
+
+	/**
+	 * Parses next element in the 'in template' insertion mode.
+	 *
+	 * This internal function performs the 'in template' insertion mode
+	 * logic for the generalized WP_HTML_Processor::step() function.
+	 *
+	 * @since 6.7.0 Stub implementation.
+	 *
+	 * @throws WP_HTML_Unsupported_Exception When encountering unsupported HTML input.
+	 *
+	 * @see https://html.spec.whatwg.org/#parsing-main-intemplate
+	 * @see WP_HTML_Processor::step
+	 *
+	 * @return bool Whether an element was found.
+	 */
+	private function step_in_template(): bool {
+		$token_name = $this->get_token_name();
+		$token_type = $this->get_token_type();
+		$is_closer  = $this->is_tag_closer();
+		$op_sigil   = '#tag' === $token_type ? ( $is_closer ? '-' : '+' ) : '';
+		$op         = "{$op_sigil}{$token_name}";
+
+		switch ( $op ) {
+			/*
+			 * > A character token
+			 * > A comment token
+			 * > A DOCTYPE token
+			 */
+			case '#text':
+			case '#comment':
+			case '#funky-comment':
+			case '#presumptuous-tag':
+			case 'html':
+				return $this->step_in_body();
+
+			/*
+			 * > A start tag whose tag name is one of: "base", "basefont", "bgsound", "link",
+			 * > "meta", "noframes", "script", "style", "template", "title"
+			 * > An end tag whose tag name is "template"
+			 */
+			case '+BASE':
+			case '+BASEFONT':
+			case '+BGSOUND':
+			case '+LINK':
+			case '+META':
+			case '+NOFRAMES':
+			case '+SCRIPT':
+			case '+STYLE':
+			case '+TEMPLATE':
+			case '+TITLE':
+			case '-TEMPLATE':
+				return $this->step_in_head();
+
+			/*
+			 * > A start tag whose tag name is one of: "caption", "colgroup", "tbody", "tfoot", "thead"
+			 */
+			case '+CAPTION':
+			case '+COLGROUP':
+			case '+TBODY':
+			case '+TFOOT':
+			case '+THEAD':
+				array_pop( $this->state->stack_of_template_insertion_modes );
+				$this->state->stack_of_template_insertion_modes[] = WP_HTML_Processor_State::INSERTION_MODE_IN_TABLE;
+				$this->state->insertion_mode                      = WP_HTML_Processor_State::INSERTION_MODE_IN_TABLE;
+				return $this->step( self::REPROCESS_CURRENT_NODE );
+
+			/*
+			 * > A start tag whose tag name is "col"
+			 */
+			case '+COL':
+				array_pop( $this->state->stack_of_template_insertion_modes );
+				$this->state->stack_of_template_insertion_modes[] = WP_HTML_Processor_State::INSERTION_MODE_IN_COLUMN_GROUP;
+				$this->state->insertion_mode                      = WP_HTML_Processor_State::INSERTION_MODE_IN_COLUMN_GROUP;
+				return $this->step( self::REPROCESS_CURRENT_NODE );
+
+			/*
+			 * > A start tag whose tag name is "tr"
+			 */
+			case '+TR':
+				array_pop( $this->state->stack_of_template_insertion_modes );
+				$this->state->stack_of_template_insertion_modes[] = WP_HTML_Processor_State::INSERTION_MODE_IN_TABLE_BODY;
+				$this->state->insertion_mode                      = WP_HTML_Processor_State::INSERTION_MODE_IN_TABLE_BODY;
+				return $this->step( self::REPROCESS_CURRENT_NODE );
+
+			/*
+			 * > A start tag whose tag name is one of: "td", "th"
+			 */
+			case '+TD':
+			case '+TH':
+				array_pop( $this->state->stack_of_template_insertion_modes );
+				$this->state->stack_of_template_insertion_modes[] = WP_HTML_Processor_State::INSERTION_MODE_IN_ROW;
+				$this->state->insertion_mode                      = WP_HTML_Processor_State::INSERTION_MODE_IN_ROW;
+				return $this->step( self::REPROCESS_CURRENT_NODE );
+		}
+
+		/*
+		 * > Any other start tag
+		 */
+		if ( ! $is_closer ) {
+			array_pop( $this->state->stack_of_template_insertion_modes );
+			$this->state->stack_of_template_insertion_modes[] = WP_HTML_Processor_State::INSERTION_MODE_IN_BODY;
+			$this->state->insertion_mode                      = WP_HTML_Processor_State::INSERTION_MODE_IN_BODY;
+			return $this->step( self::REPROCESS_CURRENT_NODE );
+		}
+
+		/*
+		 * > Any other end tag
+		 */
+		if ( $is_closer ) {
+			// Parse error: ignore the token.
+			return $this->step();
+		}
+
+		/*
+		 * > An end-of-file token
+		 */
+		if ( ! $this->state->stack_of_open_elements->contains( 'TEMPLATE' ) ) {
+			// Stop parsing.
+			return false;
+		}
+
+		// @todo Indicate a parse error once it's possible.
+		$this->state->stack_of_open_elements->pop_until( 'TEMPLATE' );
+		$this->state->active_formatting_elements->clear_up_to_last_marker();
+		array_pop( $this->state->stack_of_template_insertion_modes );
+		$this->reset_insertion_mode_appropriately();
+		return $this->step( self::REPROCESS_CURRENT_NODE );
+	}
+
+	/**
+	 * Parses next element in the 'after body' insertion mode.
+	 *
+	 * This internal function performs the 'after body' insertion mode
+	 * logic for the generalized WP_HTML_Processor::step() function.
+	 *
+	 * @since 6.7.0 Stub implementation.
+	 *
+	 * @throws WP_HTML_Unsupported_Exception When encountering unsupported HTML input.
+	 *
+	 * @see https://html.spec.whatwg.org/#parsing-main-afterbody
+	 * @see WP_HTML_Processor::step
+	 *
+	 * @return bool Whether an element was found.
+	 */
+	private function step_after_body(): bool {
+		$tag_name   = $this->get_token_name();
+		$token_type = $this->get_token_type();
+		$op_sigil   = '#tag' === $token_type ? ( $this->is_tag_closer() ? '-' : '+' ) : '';
+		$op         = "{$op_sigil}{$tag_name}";
+
+		switch ( $op ) {
+			/*
+			 * > A character token that is one of U+0009 CHARACTER TABULATION, U+000A LINE FEED (LF),
+			 * >   U+000C FORM FEED (FF), U+000D CARRIAGE RETURN (CR), or U+0020 SPACE
+			 *
+			 * > Process the token using the rules for the "in body" insertion mode.
+			 */
+			case '#text':
+				if ( parent::TEXT_IS_WHITESPACE === $this->text_node_classification ) {
+					return $this->step_in_body();
+				}
+				goto after_body_anything_else;
+				break;
+
+			/*
+			 * > A comment token
+			 */
+			case '#comment':
+			case '#funky-comment':
+			case '#presumptuous-tag':
+				$this->bail( 'Content outside of BODY is unsupported.' );
+				break;
+
+			/*
+			 * > A DOCTYPE token
+			 */
+			case 'html':
+				// Parse error: ignore the token.
+				return $this->step();
+
+			/*
+			 * > A start tag whose tag name is "html"
+			 */
+			case '+HTML':
+				return $this->step_in_body();
+
+			/*
+			 * > An end tag whose tag name is "html"
+			 *
+			 * > If the parser was created as part of the HTML fragment parsing algorithm,
+			 * > this is a parse error; ignore the token. (fragment case)
+			 * >
+			 * > Otherwise, switch the insertion mode to "after after body".
+			 */
+			case '-HTML':
+				if ( isset( $this->context_node ) ) {
+					return $this->step();
+				}
+
+				$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_AFTER_AFTER_BODY;
+				/*
+				 * The HTML element is not removed from the stack of open elements.
+				 * Only internal state has changed, this does not qualify as a "step"
+				 * in terms of advancing through the document to another token.
+				 * Nothing has been pushed or popped.
+				 * Proceed to parse the next item.
+				 */
+				return $this->step();
+		}
+
+		/*
+		 * > Parse error. Switch the insertion mode to "in body" and reprocess the token.
+		 */
+		after_body_anything_else:
+		$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_BODY;
+		return $this->step( self::REPROCESS_CURRENT_NODE );
+	}
+
+	/**
+	 * Parses next element in the 'in frameset' insertion mode.
+	 *
+	 * This internal function performs the 'in frameset' insertion mode
+	 * logic for the generalized WP_HTML_Processor::step() function.
+	 *
+	 * @since 6.7.0 Stub implementation.
+	 *
+	 * @throws WP_HTML_Unsupported_Exception When encountering unsupported HTML input.
+	 *
+	 * @see https://html.spec.whatwg.org/#parsing-main-inframeset
+	 * @see WP_HTML_Processor::step
+	 *
+	 * @return bool Whether an element was found.
+	 */
+	private function step_in_frameset(): bool {
+		$tag_name   = $this->get_token_name();
+		$token_type = $this->get_token_type();
+		$op_sigil   = '#tag' === $token_type ? ( $this->is_tag_closer() ? '-' : '+' ) : '';
+		$op         = "{$op_sigil}{$tag_name}";
+
+		switch ( $op ) {
+			/*
+			 * > A character token that is one of U+0009 CHARACTER TABULATION, U+000A LINE FEED (LF),
+			 * >   U+000C FORM FEED (FF), U+000D CARRIAGE RETURN (CR), or U+0020 SPACE
+			 * >
+			 * > Insert the character.
+			 *
+			 * This algorithm effectively strips non-whitespace characters from text and inserts
+			 * them under HTML. This is not supported at this time.
+			 */
+			case '#text':
+				if ( parent::TEXT_IS_WHITESPACE === $this->text_node_classification ) {
+					return $this->step_in_body();
+				}
+				$this->bail( 'Non-whitespace characters cannot be handled in frameset.' );
+				break;
+
+			/*
+			 * > A comment token
+			 */
+			case '#comment':
+			case '#funky-comment':
+			case '#presumptuous-tag':
+				$this->insert_html_element( $this->state->current_token );
+				return true;
+
+			/*
+			 * > A DOCTYPE token
+			 */
+			case 'html':
+				// Parse error: ignore the token.
+				return $this->step();
+
+			/*
+			 * > A start tag whose tag name is "html"
+			 */
+			case '+HTML':
+				return $this->step_in_body();
+
+			/*
+			 * > A start tag whose tag name is "frameset"
+			 */
+			case '+FRAMESET':
+				$this->insert_html_element( $this->state->current_token );
+				return true;
+
+			/*
+			 * > An end tag whose tag name is "frameset"
+			 */
+			case '-FRAMESET':
+				/*
+				 * > If the current node is the root html element, then this is a parse error;
+				 * > ignore the token. (fragment case)
+				 */
+				if ( $this->state->stack_of_open_elements->current_node_is( 'HTML' ) ) {
+					return $this->step();
+				}
+
+				/*
+				 * > Otherwise, pop the current node from the stack of open elements.
+				 */
+				$this->state->stack_of_open_elements->pop();
+
+				/*
+				 * > If the parser was not created as part of the HTML fragment parsing algorithm
+				 * > (fragment case), and the current node is no longer a frameset element, then
+				 * > switch the insertion mode to "after frameset".
+				 */
+				if ( ! isset( $this->context_node ) && ! $this->state->stack_of_open_elements->current_node_is( 'FRAMESET' ) ) {
+					$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_AFTER_FRAMESET;
+				}
+
+				return true;
+
+			/*
+			 * > A start tag whose tag name is "frame"
+			 *
+			 * > Insert an HTML element for the token. Immediately pop the
+			 * > current node off the stack of open elements.
+			 * >
+			 * > Acknowledge the token's self-closing flag, if it is set.
+			 */
+			case '+FRAME':
+				$this->insert_html_element( $this->state->current_token );
+				$this->state->stack_of_open_elements->pop();
+				return true;
+
+			/*
+			 * > A start tag whose tag name is "noframes"
+			 */
+			case '+NOFRAMES':
+				return $this->step_in_head();
+		}
+
+		// Parse error: ignore the token.
+		return $this->step();
+	}
+
+	/**
+	 * Parses next element in the 'after frameset' insertion mode.
+	 *
+	 * This internal function performs the 'after frameset' insertion mode
+	 * logic for the generalized WP_HTML_Processor::step() function.
+	 *
+	 * @since 6.7.0 Stub implementation.
+	 *
+	 * @throws WP_HTML_Unsupported_Exception When encountering unsupported HTML input.
+	 *
+	 * @see https://html.spec.whatwg.org/#parsing-main-afterframeset
+	 * @see WP_HTML_Processor::step
+	 *
+	 * @return bool Whether an element was found.
+	 */
+	private function step_after_frameset(): bool {
+		$tag_name   = $this->get_token_name();
+		$token_type = $this->get_token_type();
+		$op_sigil   = '#tag' === $token_type ? ( $this->is_tag_closer() ? '-' : '+' ) : '';
+		$op         = "{$op_sigil}{$tag_name}";
+
+		switch ( $op ) {
+			/*
+			 * > A character token that is one of U+0009 CHARACTER TABULATION, U+000A LINE FEED (LF),
+			 * >   U+000C FORM FEED (FF), U+000D CARRIAGE RETURN (CR), or U+0020 SPACE
+			 * >
+			 * > Insert the character.
+			 *
+			 * This algorithm effectively strips non-whitespace characters from text and inserts
+			 * them under HTML. This is not supported at this time.
+			 */
+			case '#text':
+				if ( parent::TEXT_IS_WHITESPACE === $this->text_node_classification ) {
+					return $this->step_in_body();
+				}
+				$this->bail( 'Non-whitespace characters cannot be handled in after frameset' );
+				break;
+
+			/*
+			 * > A comment token
+			 */
+			case '#comment':
+			case '#funky-comment':
+			case '#presumptuous-tag':
+				$this->insert_html_element( $this->state->current_token );
+				return true;
+
+			/*
+			 * > A DOCTYPE token
+			 */
+			case 'html':
+				// Parse error: ignore the token.
+				return $this->step();
+
+			/*
+			 * > A start tag whose tag name is "html"
+			 */
+			case '+HTML':
+				return $this->step_in_body();
+
+			/*
+			 * > An end tag whose tag name is "html"
+			 */
+			case '-HTML':
+				$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_AFTER_AFTER_FRAMESET;
+				/*
+				 * The HTML element is not removed from the stack of open elements.
+				 * Only internal state has changed, this does not qualify as a "step"
+				 * in terms of advancing through the document to another token.
+				 * Nothing has been pushed or popped.
+				 * Proceed to parse the next item.
+				 */
+				return $this->step();
+
+			/*
+			 * > A start tag whose tag name is "noframes"
+			 */
+			case '+NOFRAMES':
+				return $this->step_in_head();
+		}
+
+		// Parse error: ignore the token.
+		return $this->step();
+	}
+
+	/**
+	 * Parses next element in the 'after after body' insertion mode.
+	 *
+	 * This internal function performs the 'after after body' insertion mode
+	 * logic for the generalized WP_HTML_Processor::step() function.
+	 *
+	 * @since 6.7.0 Stub implementation.
+	 *
+	 * @throws WP_HTML_Unsupported_Exception When encountering unsupported HTML input.
+	 *
+	 * @see https://html.spec.whatwg.org/#the-after-after-body-insertion-mode
+	 * @see WP_HTML_Processor::step
+	 *
+	 * @return bool Whether an element was found.
+	 */
+	private function step_after_after_body(): bool {
+		$tag_name   = $this->get_token_name();
+		$token_type = $this->get_token_type();
+		$op_sigil   = '#tag' === $token_type ? ( $this->is_tag_closer() ? '-' : '+' ) : '';
+		$op         = "{$op_sigil}{$tag_name}";
+
+		switch ( $op ) {
+			/*
+			 * > A comment token
+			 */
+			case '#comment':
+			case '#funky-comment':
+			case '#presumptuous-tag':
+				$this->bail( 'Content outside of HTML is unsupported.' );
+				break;
+
+			/*
+			 * > A DOCTYPE token
+			 * > A start tag whose tag name is "html"
+			 *
+			 * > Process the token using the rules for the "in body" insertion mode.
+			 */
+			case 'html':
+			case '+HTML':
+				return $this->step_in_body();
+
+			/*
+			 * > A character token that is one of U+0009 CHARACTER TABULATION, U+000A LINE FEED (LF),
+			 * >   U+000C FORM FEED (FF), U+000D CARRIAGE RETURN (CR), or U+0020 SPACE
+			 * >
+			 * > Process the token using the rules for the "in body" insertion mode.
+			 */
+			case '#text':
+				if ( parent::TEXT_IS_WHITESPACE === $this->text_node_classification ) {
+					return $this->step_in_body();
+				}
+				goto after_after_body_anything_else;
+				break;
+		}
+
+		/*
+		 * > Parse error. Switch the insertion mode to "in body" and reprocess the token.
+		 */
+		after_after_body_anything_else:
+		$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_BODY;
+		return $this->step( self::REPROCESS_CURRENT_NODE );
+	}
+
+	/**
+	 * Parses next element in the 'after after frameset' insertion mode.
+	 *
+	 * This internal function performs the 'after after frameset' insertion mode
+	 * logic for the generalized WP_HTML_Processor::step() function.
+	 *
+	 * @since 6.7.0 Stub implementation.
+	 *
+	 * @throws WP_HTML_Unsupported_Exception When encountering unsupported HTML input.
+	 *
+	 * @see https://html.spec.whatwg.org/#the-after-after-frameset-insertion-mode
+	 * @see WP_HTML_Processor::step
+	 *
+	 * @return bool Whether an element was found.
+	 */
+	private function step_after_after_frameset(): bool {
+		$tag_name   = $this->get_token_name();
+		$token_type = $this->get_token_type();
+		$op_sigil   = '#tag' === $token_type ? ( $this->is_tag_closer() ? '-' : '+' ) : '';
+		$op         = "{$op_sigil}{$tag_name}";
+
+		switch ( $op ) {
+			/*
+			 * > A comment token
+			 */
+			case '#comment':
+			case '#funky-comment':
+			case '#presumptuous-tag':
+				$this->bail( 'Content outside of HTML is unsupported.' );
+				break;
+
+			/*
+			 * > A DOCTYPE token
+			 * > A start tag whose tag name is "html"
+			 *
+			 * > Process the token using the rules for the "in body" insertion mode.
+			 */
+			case 'html':
+			case '+HTML':
+				return $this->step_in_body();
+
+			/*
+			 * > A character token that is one of U+0009 CHARACTER TABULATION, U+000A LINE FEED (LF),
+			 * >   U+000C FORM FEED (FF), U+000D CARRIAGE RETURN (CR), or U+0020 SPACE
+			 * >
+			 * > Process the token using the rules for the "in body" insertion mode.
+			 *
+			 * This algorithm effectively strips non-whitespace characters from text and inserts
+			 * them under HTML. This is not supported at this time.
+			 */
+			case '#text':
+				if ( parent::TEXT_IS_WHITESPACE === $this->text_node_classification ) {
+					return $this->step_in_body();
+				}
+				$this->bail( 'Non-whitespace characters cannot be handled in after after frameset.' );
+				break;
+
+			/*
+			 * > A start tag whose tag name is "noframes"
+			 */
+			case '+NOFRAMES':
+				return $this->step_in_head();
+		}
+
+		// Parse error: ignore the token.
+		return $this->step();
+	}
+
+	/**
+	 * Parses next element in the 'in foreign content' insertion mode.
+	 *
+	 * This internal function performs the 'in foreign content' insertion mode
+	 * logic for the generalized WP_HTML_Processor::step() function.
+	 *
+	 * @since 6.7.0 Stub implementation.
+	 *
+	 * @throws WP_HTML_Unsupported_Exception When encountering unsupported HTML input.
+	 *
+	 * @see https://html.spec.whatwg.org/#parsing-main-inforeign
+	 * @see WP_HTML_Processor::step
+	 *
+	 * @return bool Whether an element was found.
+	 */
+	private function step_in_foreign_content(): bool {
+		$tag_name   = $this->get_token_name();
+		$token_type = $this->get_token_type();
+		$op_sigil   = '#tag' === $token_type ? ( $this->is_tag_closer() ? '-' : '+' ) : '';
+		$op         = "{$op_sigil}{$tag_name}";
+
+		/*
+		 * > A start tag whose name is "font", if the token has any attributes named "color", "face", or "size"
+		 *
+		 * This section drawn out above the switch to more easily incorporate
+		 * the additional rules based on the presence of the attributes.
+		 */
+		if (
+			'+FONT' === $op &&
+			(
+				null !== $this->get_attribute( 'color' ) ||
+				null !== $this->get_attribute( 'face' ) ||
+				null !== $this->get_attribute( 'size' )
+			)
+		) {
+			$op = '+FONT with attributes';
+		}
+
+		switch ( $op ) {
+			case '#text':
+				/*
+				 * > A character token that is U+0000 NULL
+				 *
+				 * This is handled by `get_modifiable_text()`.
+				 */
+
+				/*
+				 * Whitespace-only text does not affect the frameset-ok flag.
+				 * It is probably inter-element whitespace, but it may also
+				 * contain character references which decode only to whitespace.
+				 */
+				if ( parent::TEXT_IS_GENERIC === $this->text_node_classification ) {
+					$this->state->frameset_ok = false;
+				}
+
+				$this->insert_foreign_element( $this->state->current_token, false );
+				return true;
+
+			/*
+			 * CDATA sections are alternate wrappers for text content and therefore
+			 * ought to follow the same rules as text nodes.
+			 */
+			case '#cdata-section':
+				/*
+				 * NULL bytes and whitespace do not change the frameset-ok flag.
+				 */
+				$current_token        = $this->bookmarks[ $this->state->current_token->bookmark_name ];
+				$cdata_content_start  = $current_token->start + 9;
+				$cdata_content_length = $current_token->length - 12;
+				if ( strspn( $this->html, "\0 \t\n\f\r", $cdata_content_start, $cdata_content_length ) !== $cdata_content_length ) {
+					$this->state->frameset_ok = false;
+				}
+
+				$this->insert_foreign_element( $this->state->current_token, false );
+				return true;
+
+			/*
+			 * > A comment token
+			 */
+			case '#comment':
+			case '#funky-comment':
+			case '#presumptuous-tag':
+				$this->insert_foreign_element( $this->state->current_token, false );
+				return true;
+
+			/*
+			 * > A DOCTYPE token
+			 */
+			case 'html':
+				// Parse error: ignore the token.
+				return $this->step();
+
+			/*
+			 * > A start tag whose tag name is "b", "big", "blockquote", "body", "br", "center",
+			 * > "code", "dd", "div", "dl", "dt", "em", "embed", "h1", "h2", "h3", "h4", "h5",
+			 * > "h6", "head", "hr", "i", "img", "li", "listing", "menu", "meta", "nobr", "ol",
+			 * > "p", "pre", "ruby", "s", "small", "span", "strong", "strike", "sub", "sup",
+			 * > "table", "tt", "u", "ul", "var"
+			 *
+			 * > A start tag whose name is "font", if the token has any attributes named "color", "face", or "size"
+			 *
+			 * > An end tag whose tag name is "br", "p"
+			 *
+			 * Closing BR tags are always reported by the Tag Processor as opening tags.
+			 */
+			case '+B':
+			case '+BIG':
+			case '+BLOCKQUOTE':
+			case '+BODY':
+			case '+BR':
+			case '+CENTER':
+			case '+CODE':
+			case '+DD':
+			case '+DIV':
+			case '+DL':
+			case '+DT':
+			case '+EM':
+			case '+EMBED':
+			case '+H1':
+			case '+H2':
+			case '+H3':
+			case '+H4':
+			case '+H5':
+			case '+H6':
+			case '+HEAD':
+			case '+HR':
+			case '+I':
+			case '+IMG':
+			case '+LI':
+			case '+LISTING':
+			case '+MENU':
+			case '+META':
+			case '+NOBR':
+			case '+OL':
+			case '+P':
+			case '+PRE':
+			case '+RUBY':
+			case '+S':
+			case '+SMALL':
+			case '+SPAN':
+			case '+STRONG':
+			case '+STRIKE':
+			case '+SUB':
+			case '+SUP':
+			case '+TABLE':
+			case '+TT':
+			case '+U':
+			case '+UL':
+			case '+VAR':
+			case '+FONT with attributes':
+			case '-BR':
+			case '-P':
+				// @todo Indicate a parse error once it's possible.
+				foreach ( $this->state->stack_of_open_elements->walk_up() as $current_node ) {
+					if (
+						'math' === $current_node->integration_node_type ||
+						'html' === $current_node->integration_node_type ||
+						'html' === $current_node->namespace
+					) {
+						break;
+					}
+
+					$this->state->stack_of_open_elements->pop();
+				}
+				goto in_foreign_content_process_in_current_insertion_mode;
+		}
+
+		/*
+		 * > Any other start tag
+		 */
+		if ( ! $this->is_tag_closer() ) {
+			$this->insert_foreign_element( $this->state->current_token, false );
+
+			/*
+			 * > If the token has its self-closing flag set, then run
+			 * > the appropriate steps from the following list:
+			 * >
+			 * >   ↪ the token's tag name is "script", and the new current node is in the SVG namespace
+			 * >         Acknowledge the token's self-closing flag, and then act as
+			 * >         described in the steps for a "script" end tag below.
+			 * >
+			 * >   ↪ Otherwise
+			 * >         Pop the current node off the stack of open elements and
+			 * >         acknowledge the token's self-closing flag.
+			 *
+			 * Since the rules for SCRIPT below indicate to pop the element off of the stack of
+			 * open elements, which is the same for the Otherwise condition, there's no need to
+			 * separate these checks. The difference comes when a parser operates with the scripting
+			 * flag enabled, and executes the script, which this parser does not support.
+			 */
+			if ( $this->state->current_token->has_self_closing_flag ) {
+				$this->state->stack_of_open_elements->pop();
+			}
+			return true;
+		}
+
+		/*
+		 * > An end tag whose name is "script", if the current node is an SVG script element.
+		 */
+		if ( $this->is_tag_closer() && 'SCRIPT' === $this->state->current_token->node_name && 'svg' === $this->state->current_token->namespace ) {
+			$this->state->stack_of_open_elements->pop();
+			return true;
+		}
+
+		/*
+		 * > Any other end tag
+		 */
+		if ( $this->is_tag_closer() ) {
+			$node = $this->state->stack_of_open_elements->current_node();
+			if ( $tag_name !== $node->node_name ) {
+				// @todo Indicate a parse error once it's possible.
+			}
+			in_foreign_content_end_tag_loop:
+			if ( $node === $this->state->stack_of_open_elements->at( 1 ) ) {
+				return true;
+			}
+
+			/*
+			 * > If node's tag name, converted to ASCII lowercase, is the same as the tag name
+			 * > of the token, pop elements from the stack of open elements until node has
+			 * > been popped from the stack, and then return.
+			 */
+			if ( 0 === strcasecmp( $node->node_name, $tag_name ) ) {
+				foreach ( $this->state->stack_of_open_elements->walk_up() as $item ) {
+					$this->state->stack_of_open_elements->pop();
+					if ( $node === $item ) {
+						return true;
+					}
+				}
+			}
+
+			foreach ( $this->state->stack_of_open_elements->walk_up( $node ) as $item ) {
+				$node = $item;
+				break;
+			}
+
+			if ( 'html' !== $node->namespace ) {
+				goto in_foreign_content_end_tag_loop;
+			}
+
+			in_foreign_content_process_in_current_insertion_mode:
+			switch ( $this->state->insertion_mode ) {
+				case WP_HTML_Processor_State::INSERTION_MODE_INITIAL:
+					return $this->step_initial();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_BEFORE_HTML:
+					return $this->step_before_html();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_BEFORE_HEAD:
+					return $this->step_before_head();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_IN_HEAD:
+					return $this->step_in_head();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_IN_HEAD_NOSCRIPT:
+					return $this->step_in_head_noscript();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_AFTER_HEAD:
+					return $this->step_after_head();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_IN_BODY:
+					return $this->step_in_body();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_IN_TABLE:
+					return $this->step_in_table();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_IN_TABLE_TEXT:
+					return $this->step_in_table_text();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_IN_CAPTION:
+					return $this->step_in_caption();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_IN_COLUMN_GROUP:
+					return $this->step_in_column_group();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_IN_TABLE_BODY:
+					return $this->step_in_table_body();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_IN_ROW:
+					return $this->step_in_row();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_IN_CELL:
+					return $this->step_in_cell();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_IN_SELECT:
+					return $this->step_in_select();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_IN_SELECT_IN_TABLE:
+					return $this->step_in_select_in_table();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_IN_TEMPLATE:
+					return $this->step_in_template();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_AFTER_BODY:
+					return $this->step_after_body();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_IN_FRAMESET:
+					return $this->step_in_frameset();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_AFTER_FRAMESET:
+					return $this->step_after_frameset();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_AFTER_AFTER_BODY:
+					return $this->step_after_after_body();
+
+				case WP_HTML_Processor_State::INSERTION_MODE_AFTER_AFTER_FRAMESET:
+					return $this->step_after_after_frameset();
+
+				// This should be unreachable but PHP doesn't have total type checking on switch.
+				default:
+					$this->bail( "Unaware of the requested parsing mode: '{$this->state->insertion_mode}'." );
+			}
+		}
+
+		$this->bail( 'Should not have been able to reach end of IN FOREIGN CONTENT processing. Check HTML API code.' );
+		// This unnecessary return prevents tools from inaccurately reporting type errors.
+		return false;
 	}
 
 	/*
@@ -1476,6 +5096,19 @@
 	 */
 
 	/**
+	 * Indicates the namespace of the current token, or "html" if there is none.
+	 *
+	 * @return string One of "html", "math", or "svg".
+	 */
+	public function get_namespace(): string {
+		if ( ! isset( $this->current_element ) ) {
+			return parent::get_namespace();
+		}
+
+		return $this->current_element->token->namespace;
+	}
+
+	/**
 	 * Returns the uppercase name of the matched tag.
 	 *
 	 * The semantic rules for HTML specify that certain tags be reprocessed
@@ -1496,7 +5129,7 @@
 	 *
 	 * @return string|null Name of currently matched tag in input HTML, or `null` if none found.
 	 */
-	public function get_tag() {
+	public function get_tag(): ?string {
 		if ( null !== $this->last_error ) {
 			return null;
 		}
@@ -1507,17 +5140,13 @@
 
 		$tag_name = parent::get_tag();
 
-		switch ( $tag_name ) {
-			case 'IMAGE':
-				/*
-				 * > A start tag whose tag name is "image"
-				 * > Change the token's tag name to "img" and reprocess it. (Don't ask.)
-				 */
-				return 'IMG';
-
-			default:
-				return $tag_name;
-		}
+		/*
+		 * > A start tag whose tag name is "image"
+		 * > Change the token's tag name to "img" and reprocess it. (Don't ask.)
+		 */
+		return ( 'IMAGE' === $tag_name && 'html' === $this->get_namespace() )
+			? 'IMG'
+			: $tag_name;
 	}
 
 	/**
@@ -1537,7 +5166,7 @@
 	 *
 	 * @return bool Whether the currently matched tag contains the self-closing flag.
 	 */
-	public function has_self_closing_flag() {
+	public function has_self_closing_flag(): bool {
 		return $this->is_virtual() ? false : parent::has_self_closing_flag();
 	}
 
@@ -1561,7 +5190,7 @@
 	 *
 	 * @return string|null Name of the matched token.
 	 */
-	public function get_token_name() {
+	public function get_token_name(): ?string {
 		return $this->is_virtual()
 			? $this->current_element->token->node_name
 			: parent::get_token_name();
@@ -1589,7 +5218,7 @@
 	 *
 	 * @return string|null What kind of token is matched, or null.
 	 */
-	public function get_token_type() {
+	public function get_token_type(): ?string {
 		if ( $this->is_virtual() ) {
 			/*
 			 * This logic comes from the Tag Processor.
@@ -1651,7 +5280,7 @@
 	 * @param string|bool $value The new attribute value.
 	 * @return bool Whether an attribute value was set.
 	 */
-	public function set_attribute( $name, $value ) {
+	public function set_attribute( $name, $value ): bool {
 		return $this->is_virtual() ? false : parent::set_attribute( $name, $value );
 	}
 
@@ -1663,7 +5292,7 @@
 	 * @param string $name The attribute name to remove.
 	 * @return bool Whether an attribute was removed.
 	 */
-	public function remove_attribute( $name ) {
+	public function remove_attribute( $name ): bool {
 		return $this->is_virtual() ? false : parent::remove_attribute( $name );
 	}
 
@@ -1693,7 +5322,7 @@
 	 * @param string $prefix Prefix of requested attribute names.
 	 * @return array|null List of attribute names, or `null` when no tag opener is matched.
 	 */
-	public function get_attribute_names_with_prefix( $prefix ) {
+	public function get_attribute_names_with_prefix( $prefix ): ?array {
 		return $this->is_virtual() ? null : parent::get_attribute_names_with_prefix( $prefix );
 	}
 
@@ -1705,7 +5334,7 @@
 	 * @param string $class_name The class name to add.
 	 * @return bool Whether the class was set to be added.
 	 */
-	public function add_class( $class_name ) {
+	public function add_class( $class_name ): bool {
 		return $this->is_virtual() ? false : parent::add_class( $class_name );
 	}
 
@@ -1717,7 +5346,7 @@
 	 * @param string $class_name The class name to remove.
 	 * @return bool Whether the class was set to be removed.
 	 */
-	public function remove_class( $class_name ) {
+	public function remove_class( $class_name ): bool {
 		return $this->is_virtual() ? false : parent::remove_class( $class_name );
 	}
 
@@ -1726,10 +5355,14 @@
 	 *
 	 * @since 6.6.0 Subclassed for the HTML Processor.
 	 *
+	 * @todo When reconstructing active formatting elements with attributes, find a way
+	 *       to indicate if the virtually-reconstructed formatting elements contain the
+	 *       wanted class name.
+	 *
 	 * @param string $wanted_class Look for this CSS class name, ASCII case-insensitive.
 	 * @return bool|null Whether the matched tag contains the given class name, or null if not matched.
 	 */
-	public function has_class( $wanted_class ) {
+	public function has_class( $wanted_class ): ?bool {
 		return $this->is_virtual() ? null : parent::has_class( $wanted_class );
 	}
 
@@ -1773,7 +5406,7 @@
 	 *
 	 * @return string
 	 */
-	public function get_modifiable_text() {
+	public function get_modifiable_text(): string {
 		return $this->is_virtual() ? '' : parent::get_modifiable_text();
 	}
 
@@ -1796,7 +5429,7 @@
 	 *
 	 * @return string|null
 	 */
-	public function get_comment_type() {
+	public function get_comment_type(): ?string {
 		return $this->is_virtual() ? null : parent::get_comment_type();
 	}
 
@@ -1811,7 +5444,7 @@
 	 * @param string $bookmark_name Name of the bookmark to remove.
 	 * @return bool Whether the bookmark already existed before removal.
 	 */
-	public function release_bookmark( $bookmark_name ) {
+	public function release_bookmark( $bookmark_name ): bool {
 		return parent::release_bookmark( "_{$bookmark_name}" );
 	}
 
@@ -1832,7 +5465,7 @@
 	 * @param string $bookmark_name Jump to the place in the document identified by this bookmark name.
 	 * @return bool Whether the internal cursor was successfully moved to the bookmark's location.
 	 */
-	public function seek( $bookmark_name ) {
+	public function seek( $bookmark_name ): bool {
 		// Flush any pending updates to the document before beginning.
 		$this->get_updated_html();
 
@@ -1841,7 +5474,6 @@
 			? $this->bookmarks[ $this->state->current_token->bookmark_name ]->start
 			: 0;
 		$bookmark_starts_at   = $this->bookmarks[ $actual_bookmark_name ]->start;
-		$bookmark_length      = $this->bookmarks[ $actual_bookmark_name ]->length;
 		$direction            = $bookmark_starts_at > $processor_started_at ? 'forward' : 'backward';
 
 		/*
@@ -1874,46 +5506,92 @@
 		 * and computation time.
 		 */
 		if ( 'backward' === $direction ) {
-			/*
-			 * Instead of clearing the parser state and starting fresh, calling the stack methods
-			 * maintains the proper flags in the parser.
+
+			/*
+			 * When moving backward, stateful stacks should be cleared.
 			 */
 			foreach ( $this->state->stack_of_open_elements->walk_up() as $item ) {
-				if ( 'context-node' === $item->bookmark_name ) {
-					break;
-				}
-
 				$this->state->stack_of_open_elements->remove_node( $item );
 			}
 
 			foreach ( $this->state->active_formatting_elements->walk_up() as $item ) {
-				if ( 'context-node' === $item->bookmark_name ) {
-					break;
-				}
-
 				$this->state->active_formatting_elements->remove_node( $item );
 			}
 
-			parent::seek( 'context-node' );
-			$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_BODY;
-			$this->state->frameset_ok    = true;
-			$this->element_queue         = array();
-			$this->current_element       = null;
+			/*
+			 * **After** clearing stacks, more processor state can be reset.
+			 * This must be done after clearing the stack because those stacks generate events that
+			 * would appear on a subsequent call to `next_token()`.
+			 */
+			$this->state->frameset_ok                       = true;
+			$this->state->stack_of_template_insertion_modes = array();
+			$this->state->head_element                      = null;
+			$this->state->form_element                      = null;
+			$this->state->current_token                     = null;
+			$this->current_element                          = null;
+			$this->element_queue                            = array();
+
+			/*
+			 * The absence of a context node indicates a full parse.
+			 * The presence of a context node indicates a fragment parser.
+			 */
+			if ( null === $this->context_node ) {
+				$this->change_parsing_namespace( 'html' );
+				$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_INITIAL;
+				$this->breadcrumbs           = array();
+
+				$this->bookmarks['initial'] = new WP_HTML_Span( 0, 0 );
+				parent::seek( 'initial' );
+				unset( $this->bookmarks['initial'] );
+			} else {
+
+				/*
+				 * Push the root-node (HTML) back onto the stack of open elements.
+				 *
+				 * Fragment parsers require this extra bit of setup.
+				 * It's handled in full parsers by advancing the processor state.
+				 */
+				$this->state->stack_of_open_elements->push(
+					new WP_HTML_Token(
+						'root-node',
+						'HTML',
+						false
+					)
+				);
+
+				$this->change_parsing_namespace(
+					$this->context_node->integration_node_type
+						? 'html'
+						: $this->context_node->namespace
+				);
+
+				if ( 'TEMPLATE' === $this->context_node->node_name ) {
+					$this->state->stack_of_template_insertion_modes[] = WP_HTML_Processor_State::INSERTION_MODE_IN_TEMPLATE;
+				}
+
+				$this->reset_insertion_mode_appropriately();
+				$this->breadcrumbs = array_slice( $this->breadcrumbs, 0, 2 );
+				parent::seek( $this->context_node->bookmark_name );
+			}
 		}
 
-		// When moving forwards, reparse the document until reaching the same location as the original bookmark.
-		if ( $bookmark_starts_at === $this->bookmarks[ $this->state->current_token->bookmark_name ]->start ) {
-			return true;
-		}
-
-		while ( $this->next_token() ) {
+		/*
+		 * Here, the processor moves forward through the document until it matches the bookmark.
+		 * do-while is used here because the processor is expected to already be stopped on
+		 * a token than may match the bookmarked location.
+		 */
+		do {
+			/*
+			 * The processor will stop on virtual tokens, but bookmarks may not be set on them.
+			 * They should not be matched when seeking a bookmark, skip them.
+			 */
+			if ( $this->is_virtual() ) {
+				continue;
+			}
 			if ( $bookmark_starts_at === $this->bookmarks[ $this->state->current_token->bookmark_name ]->start ) {
-				while ( isset( $this->current_element ) && WP_HTML_Stack_Event::POP === $this->current_element->operation ) {
-					$this->current_element = array_shift( $this->element_queue );
-				}
 				return true;
 			}
-		}
+		} while ( $this->next_token() );
 
 		return false;
 	}
@@ -1993,12 +5671,25 @@
 	 * reaching for it, as inappropriate use could lead to broken
 	 * HTML structure or unwanted processing overhead.
 	 *
+	 * Bookmarks cannot be set on tokens that do no appear in the original
+	 * HTML text. For example, the HTML `<table><td>` stops at tags `TABLE`,
+	 * `TBODY`, `TR`, and `TD`. The `TBODY` and `TR` tags do not appear in
+	 * the original HTML and cannot be used as bookmarks.
+	 *
 	 * @since 6.4.0
 	 *
 	 * @param string $bookmark_name Identifies this particular bookmark.
 	 * @return bool Whether the bookmark was successfully created.
 	 */
-	public function set_bookmark( $bookmark_name ) {
+	public function set_bookmark( $bookmark_name ): bool {
+		if ( $this->is_virtual() ) {
+			_doing_it_wrong(
+				__METHOD__,
+				__( 'Cannot set bookmarks on tokens that do no appear in the original HTML text.' ),
+				'6.8.0'
+			);
+			return false;
+		}
 		return parent::set_bookmark( "_{$bookmark_name}" );
 	}
 
@@ -2010,7 +5701,7 @@
 	 * @param string $bookmark_name Name to identify a bookmark that potentially exists.
 	 * @return bool Whether that bookmark exists.
 	 */
-	public function has_bookmark( $bookmark_name ) {
+	public function has_bookmark( $bookmark_name ): bool {
 		return parent::has_bookmark( "_{$bookmark_name}" );
 	}
 
@@ -2027,7 +5718,7 @@
 	 *
 	 * @see https://html.spec.whatwg.org/#close-a-p-element
 	 */
-	private function close_a_p_element() {
+	private function close_a_p_element(): void {
 		$this->generate_implied_end_tags( 'P' );
 		$this->state->stack_of_open_elements->pop_until( 'P' );
 	}
@@ -2036,23 +5727,31 @@
 	 * Closes elements that have implied end tags.
 	 *
 	 * @since 6.4.0
+	 * @since 6.7.0 Full spec support.
 	 *
 	 * @see https://html.spec.whatwg.org/#generate-implied-end-tags
 	 *
 	 * @param string|null $except_for_this_element Perform as if this element doesn't exist in the stack of open elements.
 	 */
-	private function generate_implied_end_tags( $except_for_this_element = null ) {
+	private function generate_implied_end_tags( ?string $except_for_this_element = null ): void {
 		$elements_with_implied_end_tags = array(
 			'DD',
 			'DT',
 			'LI',
+			'OPTGROUP',
+			'OPTION',
 			'P',
+			'RB',
+			'RP',
+			'RT',
+			'RTC',
 		);
 
-		$current_node = $this->state->stack_of_open_elements->current_node();
+		$no_exclusions = ! isset( $except_for_this_element );
+
 		while (
-			$current_node && $current_node->node_name !== $except_for_this_element &&
-			in_array( $this->state->stack_of_open_elements->current_node(), $elements_with_implied_end_tags, true )
+			( $no_exclusions || ! $this->state->stack_of_open_elements->current_node_is( $except_for_this_element ) ) &&
+			in_array( $this->state->stack_of_open_elements->current_node()->node_name, $elements_with_implied_end_tags, true )
 		) {
 			$this->state->stack_of_open_elements->pop();
 		}
@@ -2065,24 +5764,61 @@
 	 * different from generating end tags in the normal sense.
 	 *
 	 * @since 6.4.0
+	 * @since 6.7.0 Full spec support.
 	 *
 	 * @see WP_HTML_Processor::generate_implied_end_tags
 	 * @see https://html.spec.whatwg.org/#generate-implied-end-tags
 	 */
-	private function generate_implied_end_tags_thoroughly() {
+	private function generate_implied_end_tags_thoroughly(): void {
 		$elements_with_implied_end_tags = array(
+			'CAPTION',
+			'COLGROUP',
 			'DD',
 			'DT',
 			'LI',
+			'OPTGROUP',
+			'OPTION',
 			'P',
+			'RB',
+			'RP',
+			'RT',
+			'RTC',
+			'TBODY',
+			'TD',
+			'TFOOT',
+			'TH',
+			'THEAD',
+			'TR',
 		);
 
-		while ( in_array( $this->state->stack_of_open_elements->current_node(), $elements_with_implied_end_tags, true ) ) {
+		while ( in_array( $this->state->stack_of_open_elements->current_node()->node_name, $elements_with_implied_end_tags, true ) ) {
 			$this->state->stack_of_open_elements->pop();
 		}
 	}
 
 	/**
+	 * Returns the adjusted current node.
+	 *
+	 * > The adjusted current node is the context element if the parser was created as
+	 * > part of the HTML fragment parsing algorithm and the stack of open elements
+	 * > has only one element in it (fragment case); otherwise, the adjusted current
+	 * > node is the current node.
+	 *
+	 * @see https://html.spec.whatwg.org/#adjusted-current-node
+	 *
+	 * @since 6.7.0
+	 *
+	 * @return WP_HTML_Token|null The adjusted current node.
+	 */
+	private function get_adjusted_current_node(): ?WP_HTML_Token {
+		if ( isset( $this->context_node ) && 1 === $this->state->stack_of_open_elements->count() ) {
+			return $this->context_node;
+		}
+
+		return $this->state->stack_of_open_elements->current_node();
+	}
+
+	/**
 	 * Reconstructs the active formatting elements.
 	 *
 	 * > This has the effect of reopening all the formatting elements that were opened
@@ -2097,7 +5833,7 @@
 	 *
 	 * @return bool Whether any formatting elements needed to be reconstructed.
 	 */
-	private function reconstruct_active_formatting_elements() {
+	private function reconstruct_active_formatting_elements(): bool {
 		/*
 		 * > If there are no entries in the list of active formatting elements, then there is nothing
 		 * > to reconstruct; stop this algorithm.
@@ -2125,8 +5861,199 @@
 			return false;
 		}
 
-		$this->last_error = self::ERROR_UNSUPPORTED;
-		throw new WP_HTML_Unsupported_Exception( 'Cannot reconstruct active formatting elements when advancing and rewinding is required.' );
+		$this->bail( 'Cannot reconstruct active formatting elements when advancing and rewinding is required.' );
+	}
+
+	/**
+	 * Runs the reset the insertion mode appropriately algorithm.
+	 *
+	 * @since 6.7.0
+	 *
+	 * @see https://html.spec.whatwg.org/multipage/parsing.html#reset-the-insertion-mode-appropriately
+	 */
+	private function reset_insertion_mode_appropriately(): void {
+		// Set the first node.
+		$first_node = null;
+		foreach ( $this->state->stack_of_open_elements->walk_down() as $first_node ) {
+			break;
+		}
+
+		/*
+		 * > 1. Let _last_ be false.
+		 */
+		$last = false;
+		foreach ( $this->state->stack_of_open_elements->walk_up() as $node ) {
+			/*
+			 * > 2. Let _node_ be the last node in the stack of open elements.
+			 * > 3. _Loop_: If _node_ is the first node in the stack of open elements, then set _last_
+			 * >            to true, and, if the parser was created as part of the HTML fragment parsing
+			 * >            algorithm (fragment case), set node to the context element passed to
+			 * >            that algorithm.
+			 * > …
+			 */
+			if ( $node === $first_node ) {
+				$last = true;
+				if ( isset( $this->context_node ) ) {
+					$node = $this->context_node;
+				}
+			}
+
+			// All of the following rules are for matching HTML elements.
+			if ( 'html' !== $node->namespace ) {
+				continue;
+			}
+
+			switch ( $node->node_name ) {
+				/*
+				 * > 4. If node is a `select` element, run these substeps:
+				 * >   1. If _last_ is true, jump to the step below labeled done.
+				 * >   2. Let _ancestor_ be _node_.
+				 * >   3. _Loop_: If _ancestor_ is the first node in the stack of open elements,
+				 * >      jump to the step below labeled done.
+				 * >   4. Let ancestor be the node before ancestor in the stack of open elements.
+				 * >   …
+				 * >   7. Jump back to the step labeled _loop_.
+				 * >   8. _Done_: Switch the insertion mode to "in select" and return.
+				 */
+				case 'SELECT':
+					if ( ! $last ) {
+						foreach ( $this->state->stack_of_open_elements->walk_up( $node ) as $ancestor ) {
+							if ( 'html' !== $ancestor->namespace ) {
+								continue;
+							}
+
+							switch ( $ancestor->node_name ) {
+								/*
+								 * > 5. If _ancestor_ is a `template` node, jump to the step below
+								 * >    labeled _done_.
+								 */
+								case 'TEMPLATE':
+									break 2;
+
+								/*
+								 * > 6. If _ancestor_ is a `table` node, switch the insertion mode to
+								 * >    "in select in table" and return.
+								 */
+								case 'TABLE':
+									$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_SELECT_IN_TABLE;
+									return;
+							}
+						}
+					}
+					$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_SELECT;
+					return;
+
+				/*
+				 * > 5. If _node_ is a `td` or `th` element and _last_ is false, then switch the
+				 * >    insertion mode to "in cell" and return.
+				 */
+				case 'TD':
+				case 'TH':
+					if ( ! $last ) {
+						$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_CELL;
+						return;
+					}
+					break;
+
+					/*
+					* > 6. If _node_ is a `tr` element, then switch the insertion mode to "in row"
+					* >    and return.
+					*/
+				case 'TR':
+					$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_ROW;
+					return;
+
+				/*
+				 * > 7. If _node_ is a `tbody`, `thead`, or `tfoot` element, then switch the
+				 * >    insertion mode to "in table body" and return.
+				 */
+				case 'TBODY':
+				case 'THEAD':
+				case 'TFOOT':
+					$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_TABLE_BODY;
+					return;
+
+				/*
+				 * > 8. If _node_ is a `caption` element, then switch the insertion mode to
+				 * >    "in caption" and return.
+				 */
+				case 'CAPTION':
+					$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_CAPTION;
+					return;
+
+				/*
+				 * > 9. If _node_ is a `colgroup` element, then switch the insertion mode to
+				 * >    "in column group" and return.
+				 */
+				case 'COLGROUP':
+					$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_COLUMN_GROUP;
+					return;
+
+				/*
+				 * > 10. If _node_ is a `table` element, then switch the insertion mode to
+				 * >     "in table" and return.
+				 */
+				case 'TABLE':
+					$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_TABLE;
+					return;
+
+				/*
+				 * > 11. If _node_ is a `template` element, then switch the insertion mode to the
+				 * >     current template insertion mode and return.
+				 */
+				case 'TEMPLATE':
+					$this->state->insertion_mode = end( $this->state->stack_of_template_insertion_modes );
+					return;
+
+				/*
+				 * > 12. If _node_ is a `head` element and _last_ is false, then switch the
+				 * >     insertion mode to "in head" and return.
+				 */
+				case 'HEAD':
+					if ( ! $last ) {
+						$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_HEAD;
+						return;
+					}
+					break;
+
+				/*
+				 * > 13. If _node_ is a `body` element, then switch the insertion mode to "in body"
+				 * >     and return.
+				 */
+				case 'BODY':
+					$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_BODY;
+					return;
+
+				/*
+				 * > 14. If _node_ is a `frameset` element, then switch the insertion mode to
+				 * >     "in frameset" and return. (fragment case)
+				 */
+				case 'FRAMESET':
+					$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_FRAMESET;
+					return;
+
+				/*
+				 * > 15. If _node_ is an `html` element, run these substeps:
+				 * >     1. If the head element pointer is null, switch the insertion mode to
+				 * >        "before head" and return. (fragment case)
+				 * >     2. Otherwise, the head element pointer is not null, switch the insertion
+				 * >        mode to "after head" and return.
+				 */
+				case 'HTML':
+					$this->state->insertion_mode = isset( $this->state->head_element )
+						? WP_HTML_Processor_State::INSERTION_MODE_AFTER_HEAD
+						: WP_HTML_Processor_State::INSERTION_MODE_BEFORE_HEAD;
+					return;
+			}
+		}
+
+		/*
+		 * > 16. If _last_ is true, then switch the insertion mode to "in body"
+		 * >     and return. (fragment case)
+		 *
+		 * This is only reachable if `$last` is true, as per the fragment parsing case.
+		 */
+		$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_BODY;
 	}
 
 	/**
@@ -2138,7 +6065,7 @@
 	 *
 	 * @see https://html.spec.whatwg.org/#adoption-agency-algorithm
 	 */
-	private function run_adoption_agency_algorithm() {
+	private function run_adoption_agency_algorithm(): void {
 		$budget       = 1000;
 		$subject      = $this->get_tag();
 		$current_node = $this->state->stack_of_open_elements->current_node();
@@ -2179,8 +6106,7 @@
 
 			// > If there is no such element, then return and instead act as described in the "any other end tag" entry above.
 			if ( null === $formatting_element ) {
-				$this->last_error = self::ERROR_UNSUPPORTED;
-				throw new WP_HTML_Unsupported_Exception( 'Cannot run adoption agency when "any other end tag" is required.' );
+				$this->bail( 'Cannot run adoption agency when "any other end tag" is required.' );
 			}
 
 			// > If formatting element is not in the stack of open elements, then this is a parse error; remove the element from the list, and return.
@@ -2210,7 +6136,7 @@
 					continue;
 				}
 
-				if ( self::is_special( $item->node_name ) ) {
+				if ( self::is_special( $item ) ) {
 					$furthest_block = $item;
 					break;
 				}
@@ -2232,12 +6158,37 @@
 				}
 			}
 
-			$this->last_error = self::ERROR_UNSUPPORTED;
-			throw new WP_HTML_Unsupported_Exception( 'Cannot extract common ancestor in adoption agency algorithm.' );
+			$this->bail( 'Cannot extract common ancestor in adoption agency algorithm.' );
 		}
 
-		$this->last_error = self::ERROR_UNSUPPORTED;
-		throw new WP_HTML_Unsupported_Exception( 'Cannot run adoption agency when looping required.' );
+		$this->bail( 'Cannot run adoption agency when looping required.' );
+	}
+
+	/**
+	 * Runs the "close the cell" algorithm.
+	 *
+	 * > Where the steps above say to close the cell, they mean to run the following algorithm:
+	 * >   1. Generate implied end tags.
+	 * >   2. If the current node is not now a td element or a th element, then this is a parse error.
+	 * >   3. Pop elements from the stack of open elements stack until a td element or a th element has been popped from the stack.
+	 * >   4. Clear the list of active formatting elements up to the last marker.
+	 * >   5. Switch the insertion mode to "in row".
+	 *
+	 * @see https://html.spec.whatwg.org/multipage/parsing.html#close-the-cell
+	 *
+	 * @since 6.7.0
+	 */
+	private function close_cell(): void {
+		$this->generate_implied_end_tags();
+		// @todo Parse error if the current node is a "td" or "th" element.
+		foreach ( $this->state->stack_of_open_elements->walk_up() as $element ) {
+			$this->state->stack_of_open_elements->pop();
+			if ( 'TD' === $element->node_name || 'TH' === $element->node_name ) {
+				break;
+			}
+		}
+		$this->state->active_formatting_elements->clear_up_to_last_marker();
+		$this->state->insertion_mode = WP_HTML_Processor_State::INSERTION_MODE_IN_ROW;
 	}
 
 	/**
@@ -2249,26 +6200,178 @@
 	 *
 	 * @param WP_HTML_Token $token Name of bookmark pointing to element in original input HTML.
 	 */
-	private function insert_html_element( $token ) {
+	private function insert_html_element( WP_HTML_Token $token ): void {
 		$this->state->stack_of_open_elements->push( $token );
 	}
 
+	/**
+	 * Inserts a foreign element on to the stack of open elements.
+	 *
+	 * @since 6.7.0
+	 *
+	 * @see https://html.spec.whatwg.org/#insert-a-foreign-element
+	 *
+	 * @param WP_HTML_Token $token                     Insert this token. The token's namespace and
+	 *                                                 insertion point will be updated correctly.
+	 * @param bool          $only_add_to_element_stack Whether to skip the "insert an element at the adjusted
+	 *                                                 insertion location" algorithm when adding this element.
+	 */
+	private function insert_foreign_element( WP_HTML_Token $token, bool $only_add_to_element_stack ): void {
+		$adjusted_current_node = $this->get_adjusted_current_node();
+
+		$token->namespace = $adjusted_current_node ? $adjusted_current_node->namespace : 'html';
+
+		if ( $this->is_mathml_integration_point() ) {
+			$token->integration_node_type = 'math';
+		} elseif ( $this->is_html_integration_point() ) {
+			$token->integration_node_type = 'html';
+		}
+
+		if ( false === $only_add_to_element_stack ) {
+			/*
+			 * @todo Implement the "appropriate place for inserting a node" and the
+			 *       "insert an element at the adjusted insertion location" algorithms.
+			 *
+			 * These algorithms mostly impacts DOM tree construction and not the HTML API.
+			 * Here, there's no DOM node onto which the element will be appended, so the
+			 * parser will skip this step.
+			 *
+			 * @see https://html.spec.whatwg.org/#insert-an-element-at-the-adjusted-insertion-location
+			 */
+		}
+
+		$this->insert_html_element( $token );
+	}
+
+	/**
+	 * Inserts a virtual element on the stack of open elements.
+	 *
+	 * @since 6.7.0
+	 *
+	 * @param string      $token_name    Name of token to create and insert into the stack of open elements.
+	 * @param string|null $bookmark_name Optional. Name to give bookmark for created virtual node.
+	 *                                   Defaults to auto-creating a bookmark name.
+	 * @return WP_HTML_Token Newly-created virtual token.
+	 */
+	private function insert_virtual_node( $token_name, $bookmark_name = null ): WP_HTML_Token {
+		$here = $this->bookmarks[ $this->state->current_token->bookmark_name ];
+		$name = $bookmark_name ?? $this->bookmark_token();
+
+		$this->bookmarks[ $name ] = new WP_HTML_Span( $here->start, 0 );
+
+		$token = new WP_HTML_Token( $name, $token_name, false );
+		$this->insert_html_element( $token );
+		return $token;
+	}
+
 	/*
 	 * HTML Specification Helpers
 	 */
 
 	/**
+	 * Indicates if the current token is a MathML integration point.
+	 *
+	 * @since 6.7.0
+	 *
+	 * @see https://html.spec.whatwg.org/#mathml-text-integration-point
+	 *
+	 * @return bool Whether the current token is a MathML integration point.
+	 */
+	private function is_mathml_integration_point(): bool {
+		$current_token = $this->state->current_token;
+		if ( ! isset( $current_token ) ) {
+			return false;
+		}
+
+		if ( 'math' !== $current_token->namespace || 'M' !== $current_token->node_name[0] ) {
+			return false;
+		}
+
+		$tag_name = $current_token->node_name;
+
+		return (
+			'MI' === $tag_name ||
+			'MO' === $tag_name ||
+			'MN' === $tag_name ||
+			'MS' === $tag_name ||
+			'MTEXT' === $tag_name
+		);
+	}
+
+	/**
+	 * Indicates if the current token is an HTML integration point.
+	 *
+	 * Note that this method must be an instance method with access
+	 * to the current token, since it needs to examine the attributes
+	 * of the currently-matched tag, if it's in the MathML namespace.
+	 * Otherwise it would be required to scan the HTML and ensure that
+	 * no other accounting is overlooked.
+	 *
+	 * @since 6.7.0
+	 *
+	 * @see https://html.spec.whatwg.org/#html-integration-point
+	 *
+	 * @return bool Whether the current token is an HTML integration point.
+	 */
+	private function is_html_integration_point(): bool {
+		$current_token = $this->state->current_token;
+		if ( ! isset( $current_token ) ) {
+			return false;
+		}
+
+		if ( 'html' === $current_token->namespace ) {
+			return false;
+		}
+
+		$tag_name = $current_token->node_name;
+
+		if ( 'svg' === $current_token->namespace ) {
+			return (
+				'DESC' === $tag_name ||
+				'FOREIGNOBJECT' === $tag_name ||
+				'TITLE' === $tag_name
+			);
+		}
+
+		if ( 'math' === $current_token->namespace ) {
+			if ( 'ANNOTATION-XML' !== $tag_name ) {
+				return false;
+			}
+
+			$encoding = $this->get_attribute( 'encoding' );
+
+			return (
+				is_string( $encoding ) &&
+				(
+					0 === strcasecmp( $encoding, 'application/xhtml+xml' ) ||
+					0 === strcasecmp( $encoding, 'text/html' )
+				)
+			);
+		}
+
+		$this->bail( 'Should not have reached end of HTML Integration Point detection: check HTML API code.' );
+		// This unnecessary return prevents tools from inaccurately reporting type errors.
+		return false;
+	}
+
+	/**
 	 * Returns whether an element of a given name is in the HTML special category.
 	 *
 	 * @since 6.4.0
 	 *
 	 * @see https://html.spec.whatwg.org/#special
 	 *
-	 * @param string $tag_name Name of element to check.
+	 * @param WP_HTML_Token|string $tag_name Node to check, or only its name if in the HTML namespace.
 	 * @return bool Whether the element of the given name is in the special category.
 	 */
-	public static function is_special( $tag_name ) {
-		$tag_name = strtoupper( $tag_name );
+	public static function is_special( $tag_name ): bool {
+		if ( is_string( $tag_name ) ) {
+			$tag_name = strtoupper( $tag_name );
+		} else {
+			$tag_name = 'html' === $tag_name->namespace
+				? strtoupper( $tag_name->node_name )
+				: "{$tag_name->namespace} {$tag_name->node_name}";
+		}
 
 		return (
 			'ADDRESS' === $tag_name ||
@@ -2356,17 +6459,17 @@
 			'XMP' === $tag_name ||
 
 			// MathML.
-			'MI' === $tag_name ||
-			'MO' === $tag_name ||
-			'MN' === $tag_name ||
-			'MS' === $tag_name ||
-			'MTEXT' === $tag_name ||
-			'ANNOTATION-XML' === $tag_name ||
+			'math MI' === $tag_name ||
+			'math MO' === $tag_name ||
+			'math MN' === $tag_name ||
+			'math MS' === $tag_name ||
+			'math MTEXT' === $tag_name ||
+			'math ANNOTATION-XML' === $tag_name ||
 
 			// SVG.
-			'FOREIGNOBJECT' === $tag_name ||
-			'DESC' === $tag_name ||
-			'TITLE' === $tag_name
+			'svg DESC' === $tag_name ||
+			'svg FOREIGNOBJECT' === $tag_name ||
+			'svg TITLE' === $tag_name
 		);
 	}
 
@@ -2382,7 +6485,7 @@
 	 * @param string $tag_name Name of HTML tag to check.
 	 * @return bool Whether the given tag is an HTML Void Element.
 	 */
-	public static function is_void( $tag_name ) {
+	public static function is_void( $tag_name ): bool {
 		$tag_name = strtoupper( $tag_name );
 
 		return (
@@ -2407,6 +6510,53 @@
 		);
 	}
 
+	/**
+	 * Gets an encoding from a given string.
+	 *
+	 * This is an algorithm defined in the WHAT-WG specification.
+	 *
+	 * Example:
+	 *
+	 *     'UTF-8' === self::get_encoding( 'utf8' );
+	 *     'UTF-8' === self::get_encoding( "  \tUTF-8 " );
+	 *     null    === self::get_encoding( 'UTF-7' );
+	 *     null    === self::get_encoding( 'utf8; charset=' );
+	 *
+	 * @see https://encoding.spec.whatwg.org/#concept-encoding-get
+	 *
+	 * @todo As this parser only supports UTF-8, only the UTF-8
+	 *       encodings are detected. Add more as desired, but the
+	 *       parser will bail on non-UTF-8 encodings.
+	 *
+	 * @since 6.7.0
+	 *
+	 * @param string $label A string which may specify a known encoding.
+	 * @return string|null Known encoding if matched, otherwise null.
+	 */
+	protected static function get_encoding( string $label ): ?string {
+		/*
+		 * > Remove any leading and trailing ASCII whitespace from label.
+		 */
+		$label = trim( $label, " \t\f\r\n" );
+
+		/*
+		 * > If label is an ASCII case-insensitive match for any of the labels listed in the
+		 * > table below, then return the corresponding encoding; otherwise return failure.
+		 */
+		switch ( strtolower( $label ) ) {
+			case 'unicode-1-1-utf-8':
+			case 'unicode11utf8':
+			case 'unicode20utf8':
+			case 'utf-8':
+			case 'utf8':
+			case 'x-unicode20utf8':
+				return 'UTF-8';
+
+			default:
+				return null;
+		}
+	}
+
 	/*
 	 * Constants that would pollute the top of the class if they were found there.
 	 */