diff -r c7c34916027a -r 177826044cd9 wp/wp-includes/widgets/class-wp-widget-media-gallery.php
--- a/wp/wp-includes/widgets/class-wp-widget-media-gallery.php	Mon Oct 14 18:06:33 2019 +0200
+++ b/wp/wp-includes/widgets/class-wp-widget-media-gallery.php	Mon Oct 14 18:28:13 2019 +0200
@@ -22,17 +22,24 @@
 	 * @since 4.9.0
 	 */
 	public function __construct() {
-		parent::__construct( 'media_gallery', __( 'Gallery' ), array(
-			'description' => __( 'Displays an image gallery.' ),
-			'mime_type'   => 'image',
-		) );
+		parent::__construct(
+			'media_gallery',
+			__( 'Gallery' ),
+			array(
+				'description' => __( 'Displays an image gallery.' ),
+				'mime_type'   => 'image',
+			)
+		);
 
-		$this->l10n = array_merge( $this->l10n, array(
-			'no_media_selected' => __( 'No images selected' ),
-			'add_media' => _x( 'Add Images', 'label for button in the gallery widget; should not be longer than ~13 characters long' ),
-			'replace_media' => '',
-			'edit_media' => _x( 'Edit Gallery', 'label for button in the gallery widget; should not be longer than ~13 characters long' ),
-		) );
+		$this->l10n = array_merge(
+			$this->l10n,
+			array(
+				'no_media_selected' => __( 'No images selected' ),
+				'add_media'         => _x( 'Add Images', 'label for button in the gallery widget; should not be longer than ~13 characters long' ),
+				'replace_media'     => '',
+				'edit_media'        => _x( 'Edit Gallery', 'label for button in the gallery widget; should not be longer than ~13 characters long' ),
+			)
+		);
 	}
 
 	/**
@@ -47,37 +54,37 @@
 	 */
 	public function get_instance_schema() {
 		$schema = array(
-			'title' => array(
-				'type' => 'string',
-				'default' => '',
-				'sanitize_callback' => 'sanitize_text_field',
-				'description' => __( 'Title for the widget' ),
+			'title'          => array(
+				'type'                  => 'string',
+				'default'               => '',
+				'sanitize_callback'     => 'sanitize_text_field',
+				'description'           => __( 'Title for the widget' ),
 				'should_preview_update' => false,
 			),
-			'ids' => array(
-				'type' => 'array',
-				'items' => array(
+			'ids'            => array(
+				'type'              => 'array',
+				'items'             => array(
 					'type' => 'integer',
 				),
-				'default' => array(),
+				'default'           => array(),
 				'sanitize_callback' => 'wp_parse_id_list',
 			),
-			'columns' => array(
-				'type' => 'integer',
+			'columns'        => array(
+				'type'    => 'integer',
 				'default' => 3,
 				'minimum' => 1,
 				'maximum' => 9,
 			),
-			'size' => array(
-				'type' => 'string',
-				'enum' => array_merge( get_intermediate_image_sizes(), array( 'full', 'custom' ) ),
+			'size'           => array(
+				'type'    => 'string',
+				'enum'    => array_merge( get_intermediate_image_sizes(), array( 'full', 'custom' ) ),
 				'default' => 'thumbnail',
 			),
-			'link_type' => array(
-				'type' => 'string',
-				'enum' => array( 'post', 'file', 'none' ),
-				'default' => 'post',
-				'media_prop' => 'link',
+			'link_type'      => array(
+				'type'                  => 'string',
+				'enum'                  => array( 'post', 'file', 'none' ),
+				'default'               => 'post',
+				'media_prop'            => 'link',
 				'should_preview_update' => false,
 			),
 			'orderby_random' => array(
@@ -168,37 +175,60 @@
 		parent::render_control_template_scripts();
 		?>
 		<script type="text/html" id="tmpl-wp-media-widget-gallery-preview">
-			<# var describedById = 'describedBy-' + String( Math.random() ); #>
-			<# if ( data.ids.length ) { #>
-				<div class="gallery media-widget-gallery-preview">
-					<# _.each( data.ids, function( id, index ) { #>
-						<#
-						var attachment = data.attachments[ id ];
-						if ( ! attachment ) {
-							return;
-						}
-						#>
+			<#
+			var ids = _.filter( data.ids, function( id ) {
+				return ( id in data.attachments );
+			} );
+			#>
+			<# if ( ids.length ) { #>
+				<ul class="gallery media-widget-gallery-preview" role="list">
+					<# _.each( ids, function( id, index ) { #>
+						<# var attachment = data.attachments[ id ]; #>
 						<# if ( index < 6 ) { #>
-							<dl class="gallery-item">
-								<dt class="gallery-icon">
-								<# if ( attachment.sizes.thumbnail ) { #>
-									<img src="{{ attachment.sizes.thumbnail.url }}" width="{{ attachment.sizes.thumbnail.width }}" height="{{ attachment.sizes.thumbnail.height }}" alt="" />
-								<# } else { #>
-									<img src="{{ attachment.url }}" alt="" />
-								<# } #>
-								<# if ( index === 5 && data.ids.length > 6 ) { #>
+							<li class="gallery-item">
+								<div class="gallery-icon">
+									<img alt="{{ attachment.alt }}"
+										<# if ( index === 5 && data.ids.length > 6 ) { #> aria-hidden="true" <# } #>
+										<# if ( attachment.sizes.thumbnail ) { #>
+											src="{{ attachment.sizes.thumbnail.url }}" width="{{ attachment.sizes.thumbnail.width }}" height="{{ attachment.sizes.thumbnail.height }}"
+										<# } else { #>
+											src="{{ attachment.url }}"
+										<# } #>
+										<# if ( ! attachment.alt && attachment.filename ) { #>
+											aria-label="
+											<?php
+											echo esc_attr(
+												sprintf(
+													/* translators: %s: the image file name. */
+													__( 'The current image has no alternative text. The file name is: %s' ),
+													'{{ attachment.filename }}'
+												)
+											);
+											?>
+											"
+										<# } #>
+									/>
+									<# if ( index === 5 && data.ids.length > 6 ) { #>
 									<div class="gallery-icon-placeholder">
-										<p class="gallery-icon-placeholder-text">+{{ data.ids.length - 5 }}</p>
+										<p class="gallery-icon-placeholder-text" aria-label="
+										<?php
+											printf(
+												/* translators: %s: the amount of additional, not visible images in the gallery widget preview. */
+												__( 'Additional images added to this gallery: %s' ),
+												'{{ data.ids.length - 5 }}'
+											);
+										?>
+										">+{{ data.ids.length - 5 }}</p>
 									</div>
-								<# } #>
-								</dt>
-							</dl>
+									<# } #>
+								</div>
+							</li>
 						<# } #>
 					<# } ); #>
-				</div>
+				</ul>
 			<# } else { #>
 				<div class="attachment-media-view">
-					<p class="placeholder"><?php echo esc_html( $this->l10n['no_media_selected'] ); ?></p>
+					<button type="button" class="placeholder button-add-media"><?php echo esc_html( $this->l10n['add_media'] ); ?></button>
 				</div>
 			<# } #>
 		</script>