diff --git a/wp-includes/html-api/class-wp-html-open-elements.php b/wp-includes/html-api/class-wp-html-open-elements.php
index fe5625545b..55c4d3a663 100644
--- a/wp-includes/html-api/class-wp-html-open-elements.php
+++ b/wp-includes/html-api/class-wp-html-open-elements.php
@@ -116,6 +116,13 @@ class WP_HTML_Open_Elements {
 				return true;
 			}
 
+			if (
+				'(internal: H1 through H6 - do not use)' === $tag_name &&
+				in_array( $node->node_name, array( 'H1', 'H2', 'H3', 'H4', 'H5', 'H6' ), true )
+			) {
+				return true;
+			}
+
 			switch ( $node->node_name ) {
 				case 'HTML':
 					return false;
@@ -270,6 +277,13 @@ class WP_HTML_Open_Elements {
 		foreach ( $this->walk_up() as $item ) {
 			$this->pop();
 
+			if (
+				'(internal: H1 through H6 - do not use)' === $tag_name &&
+				in_array( $item->node_name, array( 'H1', 'H2', 'H3', 'H4', 'H5', 'H6' ), true )
+			) {
+				return true;
+			}
+
 			if ( $tag_name === $item->node_name ) {
 				return true;
 			}
diff --git a/wp-includes/html-api/class-wp-html-processor.php b/wp-includes/html-api/class-wp-html-processor.php
index 779c7afec3..14dfc3aa86 100644
--- a/wp-includes/html-api/class-wp-html-processor.php
+++ b/wp-includes/html-api/class-wp-html-processor.php
@@ -102,7 +102,7 @@
  *  - Containers: ADDRESS, BLOCKQUOTE, DETAILS, DIALOG, DIV, FOOTER, HEADER, MAIN, MENU, SPAN, SUMMARY.
  *  - Form elements: BUTTON, FIELDSET, SEARCH.
  *  - Formatting elements: B, BIG, CODE, EM, FONT, I, SMALL, STRIKE, STRONG, TT, U.
- *  - Heading elements: HGROUP.
+ *  - Heading elements: H1, H2, H3, H4, H5, H6, HGROUP.
  *  - Links: A.
  *  - Lists: DL.
  *  - Media elements: FIGCAPTION, FIGURE, IMG.
@@ -697,6 +697,60 @@ class WP_HTML_Processor extends WP_HTML_Tag_Processor {
 				$this->state->stack_of_open_elements->pop_until( $tag_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;
+
+			/*
+			 * > 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->generate_implied_end_tags();
+
+				if ( $this->state->stack_of_open_elements->current_node()->node_name !== $tag_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;
+
 			/*
 			 * > An end tag whose tag name is "p"
 			 */
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 91eeea0f78..6648723d44 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.5-alpha-57185';
+$wp_version = '6.5-alpha-57186';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.