diff -r 34716fd837a4 -r be944660c56a wp/wp-includes/block-supports/custom-classname.php
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/wp/wp-includes/block-supports/custom-classname.php	Wed Sep 21 18:19:35 2022 +0200
@@ -0,0 +1,65 @@
+<?php
+/**
+ * Custom classname block support flag.
+ *
+ * @package WordPress
+ * @since 5.6.0
+ */
+
+/**
+ * Registers the custom classname block attribute for block types that support it.
+ *
+ * @since 5.6.0
+ * @access private
+ *
+ * @param WP_Block_Type $block_type Block Type.
+ */
+function wp_register_custom_classname_support( $block_type ) {
+	$has_custom_classname_support = block_has_support( $block_type, array( 'customClassName' ), true );
+
+	if ( $has_custom_classname_support ) {
+		if ( ! $block_type->attributes ) {
+			$block_type->attributes = array();
+		}
+
+		if ( ! array_key_exists( 'className', $block_type->attributes ) ) {
+			$block_type->attributes['className'] = array(
+				'type' => 'string',
+			);
+		}
+	}
+}
+
+/**
+ * Add the custom classnames to the output.
+ *
+ * @since 5.6.0
+ * @access private
+ *
+ * @param  WP_Block_Type $block_type       Block Type.
+ * @param  array         $block_attributes Block attributes.
+ *
+ * @return array Block CSS classes and inline styles.
+ */
+function wp_apply_custom_classname_support( $block_type, $block_attributes ) {
+	$has_custom_classname_support = block_has_support( $block_type, array( 'customClassName' ), true );
+	$attributes                   = array();
+	if ( $has_custom_classname_support ) {
+		$has_custom_classnames = array_key_exists( 'className', $block_attributes );
+
+		if ( $has_custom_classnames ) {
+			$attributes['class'] = $block_attributes['className'];
+		}
+	}
+
+	return $attributes;
+}
+
+// Register the block support.
+WP_Block_Supports::get_instance()->register(
+	'custom-classname',
+	array(
+		'register_attribute' => 'wp_register_custom_classname_support',
+		'apply'              => 'wp_apply_custom_classname_support',
+	)
+);