diff --git a/src/wp-includes/html-api/class-wp-html-tag-processor.php b/src/wp-includes/html-api/class-wp-html-tag-processor.php
index 61b0cb696f186..bf0a881e6c756 100644
--- a/src/wp-includes/html-api/class-wp-html-tag-processor.php
+++ b/src/wp-includes/html-api/class-wp-html-tag-processor.php
@@ -317,6 +317,16 @@ class WP_HTML_Tag_Processor {
 	 */
 	private $stop_on_tag_closers;
 
+	/**
+	 * Whether to visit funky comments, e.g. </1>, when walking an input document.
+	 *
+	 * These are funny because they are errors.
+	 *
+	 * @since 6.3.0
+	 * @var bool
+	 */
+	private $stop_on_funky_comments;
+
 	/**
 	 * Holds updated HTML as updates are applied.
 	 *
@@ -538,6 +548,18 @@ class WP_HTML_Tag_Processor {
 	 */
 	protected $seek_count = 0;
 
+	/**
+	 * @since 6.3.0
+	 * @var string
+	 */
+	private $funky_comment_content = null;
+
+	/**
+	 * @since 6.3.0
+	 * @var int
+	 */
+	private $placeholders = 0;
+
 	/**
 	 * Constructor.
 	 *
@@ -1161,11 +1183,21 @@ private function parse_next_tag() {
 			 * See https://github.com/WordPress/wordpress-develop/pull/4256
 			 */
 			if ( $this->is_closing_tag ) {
-				$closer_at = strpos( $html, '>', $at + 3 );
+				$closer_at = strpos( $html, '>', $at );
 				if ( false === $closer_at ) {
 					return false;
 				}
 
+				if ( $this->stop_on_funky_comments ) {
+					++$at;
+					$this->tag_name_length       = 0;
+					$this->tag_name_starts_at    = $at;
+					$this->bytes_already_parsed  = $closer_at;
+					$this->funky_comment_content = array( $at, $closer_at );
+
+					return true;
+				}
+
 				$at = $closer_at + 1;
 				continue;
 			}
@@ -1301,11 +1333,12 @@ private function skip_whitespace() {
 	private function after_tag() {
 		$this->class_name_updates_to_attributes_updates();
 		$this->apply_attributes_updates();
-		$this->tag_name_starts_at = null;
-		$this->tag_name_length    = null;
-		$this->tag_ends_at        = null;
-		$this->is_closing_tag     = null;
-		$this->attributes         = array();
+		$this->tag_name_starts_at    = null;
+		$this->tag_name_length       = null;
+		$this->tag_ends_at           = null;
+		$this->is_closing_tag        = null;
+		$this->attributes            = array();
+		$this->funky_comment_content = null;
 	}
 
 	/**
@@ -1552,7 +1585,7 @@ public function seek( $bookmark_name ) {
 		if ( ! array_key_exists( $bookmark_name, $this->bookmarks ) ) {
 			_doing_it_wrong(
 				__METHOD__,
-				__( 'Unknown bookmark name.' ),
+				__( 'Unknown bookmark name.' . ' ' . $bookmark_name ),
 				'6.2.0'
 			);
 			return false;
@@ -1577,6 +1610,14 @@ public function seek( $bookmark_name ) {
 		return $this->next_tag( array( 'tag_closers' => 'visit' ) );
 	}
 
+	public function rewind() {
+//		$this->get_updated_html();
+		$this->after_tag();
+		$this->bytes_already_parsed = 0;
+		$this->bytes_already_copied = 0;
+		$this->output_buffer        = '';
+	}
+
 	/**
 	 * Compare two WP_HTML_Text_Replacement objects.
 	 *
@@ -1857,6 +1898,13 @@ public function is_tag_closer() {
 		return $this->is_closing_tag;
 	}
 
+	/**
+	 * @since 6.3.0
+	 */
+	public function is_funky_comment() {
+		return null !== $this->funky_comment_content;
+	}
+
 	/**
 	 * Updates or creates a new attribute on the currently matched tag with the passed value.
 	 *
@@ -2113,6 +2161,13 @@ public function __toString() {
 		return $this->get_updated_html();
 	}
 
+	public function get_funky_content() {
+		if ( $this->funky_comment_content !== null ) {
+			list( $start, $end ) = $this->funky_comment_content;
+			return substr( $this->html, $start, $end - $start );
+		}
+	}
+
 	/**
 	 * Returns the string representation of the HTML Tag Processor.
 	 *
@@ -2204,11 +2259,12 @@ private function parse_query( $query ) {
 			return;
 		}
 
-		$this->last_query          = $query;
-		$this->sought_tag_name     = null;
-		$this->sought_class_name   = null;
-		$this->sought_match_offset = 1;
-		$this->stop_on_tag_closers = false;
+		$this->last_query             = $query;
+		$this->sought_tag_name        = null;
+		$this->sought_class_name      = null;
+		$this->sought_match_offset    = 1;
+		$this->stop_on_tag_closers    = false;
+		$this->stop_on_funky_comments = false;
 
 		// A single string value means "find the tag of this name".
 		if ( is_string( $query ) ) {
@@ -2246,8 +2302,94 @@ private function parse_query( $query ) {
 		if ( isset( $query['tag_closers'] ) ) {
 			$this->stop_on_tag_closers = 'visit' === $query['tag_closers'];
 		}
+
+		if ( isset( $query['funky_comments'] ) ) {
+			$this->stop_on_funky_comments = 'visit' === $query['funky_comments'];
+		}
 	}
 
+	public function declarative_match( $pattern_html ) {
+		$this->placeholders = 0;
+		while ( $this->placeholders > 0 ) {
+			$this->release_bookmark( "__placeholder_{$this->placeholders}" );
+			$this->placeholders--;
+		}
+		$pattern = new WP_HTML_Tag_Processor( $pattern_html );
+		$visit_everything = array( 'tag_closers' => 'visit', 'funky_comments' => 'visit' );
+
+		$same_thing = function ( WP_HTML_Tag_Processor $pattern, WP_HTML_Tag_Processor $test ) {
+			if ( $pattern->is_funky_comment() ) {
+				$this->placeholders++;
+				$this->set_bookmark( "__placeholder_{$this->placeholders}" );
+				return true;
+			}
+
+			if ( ! (
+				$pattern->get_tag() === $test->get_tag() &&
+				$pattern->is_tag_closer() === $test->is_tag_closer() &&
+				$pattern->is_funky_comment() === $test->is_funky_comment()
+			) ) {
+				return false;
+			}
+
+			$attribute_constraints = $pattern->get_attribute_names_with_prefix( '' );
+			if ( null === $attribute_constraints ) {
+				return true;
+			}
+
+			foreach ( $attribute_constraints as $name ) {
+				if ( $pattern->get_attribute( $name ) !== $test->get_attribute( $name ) ) {
+					return false;
+				}
+			}
+
+			return true;
+		};
+
+		step_one: // find the next spot the patterns start the same.
+		if ( ! $pattern->next_tag( $visit_everything ) ) {
+			return false;
+		}
+
+		while ( $this->placeholders > 0 ) {
+			$this->release_bookmark( "__placeholder_{$this->placeholders}" );
+			$this->placeholders--;
+		}
+		while ( $this->next_tag( $visit_everything ) ) {
+			if ( $same_thing( $pattern, $this ) ) {
+				goto step_two;
+			}
+		}
+		return false;
+
+		step_two: // see if the subsequence tokens in the pattern and test match.
+		$this->set_bookmark( 'match_start' );
+		if ( ! $pattern->next_tag( $visit_everything ) ) {
+			$this->release_bookmark( 'match_start' );
+			return true;
+		}
+
+		while ( true ) {
+			if ( ! $this->next_tag( $visit_everything ) ) {
+				$this->release_bookmark( 'match_start' );
+				return false;
+			}
+
+			if ( ! $same_thing( $pattern, $this ) ) {
+				$pattern->rewind();
+				goto step_one;
+			}
+
+			if ( ! $pattern->next_tag( $visit_everything ) ) {
+				break;
+			}
+		}
+
+		$this->set_bookmark( 'match_end' );
+		$this->seek( 'match_start' );
+		$this->release_bookmark( 'match_start' );
+		return true;
+	}
 
 	/**
 	 * Checks whether a given tag and its attributes match the search criteria.
@@ -2257,6 +2399,10 @@ private function parse_query( $query ) {
 	 * @return boolean Whether the given tag and its attribute match the search criteria.
 	 */
 	private function matches() {
+		if ( null !== $this->funky_comment_content && $this->stop_on_funky_comments ) {
+			return true;
+		}
+
 		if ( $this->is_closing_tag && ! $this->stop_on_tag_closers ) {
 			return false;
 		}
diff --git a/tests/phpunit/tests/html-api/wpHtmlTagProcessor.php b/tests/phpunit/tests/html-api/wpHtmlTagProcessor.php
index 7b74b279124ed..62b58fe5d86a6 100644
--- a/tests/phpunit/tests/html-api/wpHtmlTagProcessor.php
+++ b/tests/phpunit/tests/html-api/wpHtmlTagProcessor.php
@@ -17,6 +17,10 @@ class WP_UnitTestCase extends PHPUnit\Framework\TestCase {}
 //	require_once '/Users/dmsnell/code/WordPress-develop/src/wp-includes/html-api/class-wp-html-processor.php';
 
 	function esc_attr( $s ) { return str_replace( [ '<', '>', '"' ], [ '&lt;', '&gt;', '&quot;' ], $s ); }
+	function __( $s ) { return $s; }
+	function _doing_it_wrong( ...$args ) {
+		var_dump( $args );
+	}
 }
 
 /**
@@ -2247,4 +2251,97 @@ public function data_updating_attributes_in_malformed_html() {
 			),
 		);
 	}
+
+	/**
+	 * @dataProvider data_funky_comments
+	 */
+	public function test_stops_at_funky_comments( $html, $content ) {
+		$p = new WP_HTML_Tag_Processor( $html );
+
+		$this->assertTrue( $p->next_tag( array( 'funky_comments' => 'visit' ) ) );
+		$this->assertEquals( $content, $p->get_funky_content() );
+	}
+
+	public function data_funky_comments() {
+		return array(
+			'Isolated comment'   => array( '</1>', '1' ),
+			'Inside text'        => array( 'Before</1>After', '1' ),
+			'%name syntax'       => array( 'Today is </%day>.', '%day' ),
+			'With spaces inside' => array( 'What </$variable is this>?', '$variable is this' ),
+		);
+	}
+
+	/**
+	 * @dataProvider data_declarative_patterns
+	 */
+	public function test_matches_declarative_pattern( $pattern, $html, $matches ) {
+		$p = new WP_HTML_Tag_Processor( $html );
+
+		if ( $matches ) {
+			$this->assertTrue( $p->declarative_match( $pattern ) );
+		} else {
+			$this->assertFalse( $p->declarative_match( $pattern ) );
+		}
+	}
+
+	public function data_declarative_patterns() {
+		return array(
+			'Single tag'                 => array( '<div>', '<div>', true ),
+			'^Single tag'                => array( '<div>', '<img>', false ),
+			'Wrapped image'              => array( '<div><img></div>', '<div><img></div>', true ),
+			'Wrapped image w/attributes' => array( '<div><img></div>', '<div id="14"><img src="hallumi" inert></div>', true ),
+			'Prefix before match'        => array( '<li><img></li>', '<main><h1>Stuff!</h1><ul><li><img></li></ul></main>', true ),
+			'Pattern with attribute'     => array( '<li is-active><img></li>', '<li is-active><img></li>', true ),
+			'^Pattern with attribute'    => array( '<li is-active><img></li>', '<li><img></li>', false ),
+			'Pattern with attributes'    => array( '<li is-active class="slick"><img></li>', '<li class="slick" is-active><img></li>', true ),
+			'^Pattern with attributes'   => array( '<li is-active class="slick"><img></li>', '<li id="slick" is-active><img></li>', false ),
+			'^Pattern with attributes 2' => array( '<li is-active class="slick"><img></li>', '<li class="wicket" is-active><img></li>', false ),
+			'Test with attributes'       => array( '<li is-active><img></li>', '<li id="5" is-funky=maybe style=\'color: red;\' is-active class="test-class bright"><img></li>', true ),
+			'^Test with attributes'      => array( '<li is-active><img></li>', '<li id="5" is-funky=maybe style=\'color: red;\' isactive class="test-class bright"><img></li>', false ),
+			'Attribute with value'       => array( '<input disabled>', '<input type="text"><input><input disabled><input value="5">', true ),
+			'Attribute with text'        => array( '<input id="5">', '<input type="text"><input><input id=5><input disabled><input value="5">', true ),
+			'^Attribute with value'      => array( '<input disabled>', '<input type="text"><input><input disable><input value="5">', false ),
+			'Wildcard'                   => array( '<hgroup></1></2></hgroup>', '<hgroup><h1>Important</h1></hgroup>', true ),
+			'^Wildcard'                  => array( '<hgroup></1></2></hgroup>', '<hgroup><img></hgroup>', false ),
+			'Wildcard attributes'        => array( '</1 aria-label="placeholder">', '<div><p><strong>This</strong> is <em aria-label="placeholder">really</em> cool!</p></div>', true ),
+		);
+	}
+
+	public function test_declarative_match_pauses_at_start_of_match() {
+		$p = new WP_HTML_Tag_Processor( '<main><h1>Stuff!</h1><ul><li pick-me><img></li></ul></main>' );
+
+		$this->assertTrue( $p->declarative_match( '<li><img></li>' ) );
+		$this->assertTrue( $p->get_attribute( 'pick-me' ) );
+	}
+
+	public function test_declarative_match_bookmarks_markup_wildcards_delete_me_this_is_an_internal_detail_but_for_now_helpful_for_development() {
+		$p = new WP_HTML_Tag_Processor( <<<HTML
+			<main>
+				<h1>Stuff!</h1>
+				<ul>
+					<li id=1><p>Just a thought</p></li>
+					<img>
+					<li id=2 pick-me><img></li>
+				</ul>
+			</main>
+HTML
+		);
+
+		$p->next_tag();
+
+		$this->assertTrue( $p->declarative_match( '<li></1></li>' ) );
+		$p->seek( '__placeholder_1' );
+		$this->assertSame( 'IMG', $p->get_tag() );
+
+		$p->rewind();
+		$this->assertTrue( $p->declarative_match( '<main></1></2><ul>' ) );
+
+		$p->seek( '__placeholder_1' );
+		$this->assertSame( 'H1', $p->get_tag() );
+		$this->assertFalse( $p->is_tag_closer() );
+
+		$p->seek( '__placeholder_2' );
+		$this->assertSame( 'H1', $p->get_tag() );
+		$this->assertTrue( $p->is_tag_closer() );
+	}
 }