class WP_HTML_Processor {}

Core class used to safely parse and modify an HTML document.

Description

The HTML Processor class properly parses and modifies HTML5 documents.

It supports a subset of the HTML5 specification, and when it encounters unsupported markup, it aborts early to avoid unintentionally breaking the document. The HTML Processor should never break an HTML document.

While the WP_HTML_Tag_Processor is a valuable tool for modifying attributes on individual HTML tags, the HTML Processor is more capable and useful for the following operations:

  • Querying based on nested HTML structure.

Eventually the HTML Processor will also support:

  • Wrapping a tag in surrounding HTML.
  • Unwrapping a tag by removing its parent.
  • Inserting and removing nodes.
  • Reading and changing inner content.
  • Navigating up or around HTML structure.

Usage

Use of this class requires three steps:

  1. Call a static creator method with your input HTML document.
  2. Find the location in the document you are looking for.
  3. Request changes to the document at that location.

Example:

$processor = WP_HTML_Processor::create_fragment( $html );
if ( $processor->next_tag( array( 'breadcrumbs' => array( 'DIV', 'FIGURE', 'IMG' ) ) ) ) {
    $processor->add_class( 'responsive-image' );
}

Breadcrumbs

Breadcrumbs represent the stack of open elements from the root of the document or fragment down to the currently-matched node, if one is currently selected. Call WP_HTML_Processor::get_breadcrumbs() to inspect the breadcrumbs for a matched tag.

Breadcrumbs can specify nested HTML structure and are equivalent to a CSS selector comprising tag names separated by the child combinator, such as "DIV > FIGURE > IMG".

Since all elements find themselves inside a full HTML document when parsed, the return value from get_breadcrumbs() will always contain any implicit outermost elements. For example, when parsing with create_fragment() in the BODY context (the default), any tag in the given HTML document will contain array( 'HTML', 'BODY', … ) in its breadcrumbs.

Despite containing the implied outermost elements in their breadcrumbs, tags may be found with the shortest-matching breadcrumb query. That is, array( 'IMG' ) matches all IMG elements and array( 'P', 'IMG' ) matches all IMG elements directly inside a P element. To ensure that no partial matches erroneously match it’s possible to specify in a query the full breadcrumb match all the way down from the root HTML element.

Example:

$html = '<figure><img><figcaption>A <em>lovely</em> day outside</figcaption></figure>';
//               ----- Matches here.
$processor->next_tag( array( 'breadcrumbs' => array( 'FIGURE', 'IMG' ) ) );

$html = '<figure><img><figcaption>A <em>lovely</em> day outside</figcaption></figure>';
//                                  ---- Matches here.
$processor->next_tag( array( 'breadcrumbs' => array( 'FIGURE', 'FIGCAPTION', 'EM' ) ) );

$html = '<div><img></div><img>';
//                       ----- Matches here, because IMG must be a direct child of the implicit BODY.
$processor->next_tag( array( 'breadcrumbs' => array( 'BODY', 'IMG' ) ) );

HTML Support

This class implements a small part of the HTML5 specification.
It’s designed to operate within its support and abort early whenever encountering circumstances it can’t properly handle. This is the principle way in which this class remains as simple as possible without cutting corners and breaking compliance.

Supported elements

If any unsupported element appears in the HTML input the HTML Processor 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:

  • 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.

Supported markup

Some kinds of non-normative HTML involve reconstruction of formatting elements and re-parenting of mis-nested elements. For example, a DIV tag found inside a TABLE 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:

  • 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.

See also

Methods

NameDescription
WP_HTML_Processor::__constructConstructor.
WP_HTML_Processor::bookmark_tagCreates a new bookmark for the currently-matched tag and returns the generated name.
WP_HTML_Processor::bookmark_tokenCreates a new bookmark for the currently-matched token and returns the generated name.
WP_HTML_Processor::close_a_p_elementCloses a P element.
WP_HTML_Processor::create_fragmentCreates an HTML processor in the fragment parsing mode.
WP_HTML_Processor::generate_implied_end_tagsCloses elements that have implied end tags.
WP_HTML_Processor::generate_implied_end_tags_thoroughlyCloses elements that have implied end tags, thoroughly.
WP_HTML_Processor::get_breadcrumbsComputes the HTML breadcrumbs for the currently-matched node, if matched.
WP_HTML_Processor::get_last_errorReturns the last error, if any.
WP_HTML_Processor::get_tagReturns the uppercase name of the matched tag.
WP_HTML_Processor::has_bookmarkChecks whether a bookmark with the given name exists.
WP_HTML_Processor::insert_html_elementInserts an HTML element on the stack of open elements.
WP_HTML_Processor::is_specialReturns whether an element of a given name is in the HTML special category.
WP_HTML_Processor::is_voidReturns whether a given element is an HTML Void Element
WP_HTML_Processor::matches_breadcrumbsIndicates if the currently-matched tag matches the given breadcrumbs.
WP_HTML_Processor::next_tagFinds the next tag matching the $query.
WP_HTML_Processor::next_tokenEnsures internal accounting is maintained for HTML semantic rules while the underlying Tag Processor class is seeking to a bookmark.
WP_HTML_Processor::reconstruct_active_formatting_elementsReconstructs the active formatting elements.
WP_HTML_Processor::release_bookmarkRemoves a bookmark that is no longer needed.
WP_HTML_Processor::run_adoption_agency_algorithmRuns the adoption agency algorithm.
WP_HTML_Processor::seekMoves the internal cursor in the HTML Processor to a given bookmark’s location.
WP_HTML_Processor::set_bookmarkSets a bookmark in the HTML document.
WP_HTML_Processor::stepSteps through the HTML document and stop at the next tag, if any.
WP_HTML_Processor::step_in_bodyParses next element in the ‘in body’ insertion mode.

Source

class WP_HTML_Processor extends WP_HTML_Tag_Processor {
	/**
	 * The maximum number of bookmarks allowed to exist at any given time.
	 *
	 * HTML processing requires more bookmarks than basic tag processing,
	 * so this class constant from the Tag Processor is overwritten.
	 *
	 * @since 6.4.0
	 *
	 * @var int
	 */
	const MAX_BOOKMARKS = 100;

	/**
	 * Holds the working state of the parser, including the stack of
	 * open elements and the stack of active formatting elements.
	 *
	 * Initialized in the constructor.
	 *
	 * @since 6.4.0
	 *
	 * @var WP_HTML_Processor_State
	 */
	private $state = null;

	/**
	 * Used to create unique bookmark names.
	 *
	 * This class sets a bookmark for every tag in the HTML document that it encounters.
	 * The bookmark name is auto-generated and increments, starting with `1`. These are
	 * internal bookmarks and are automatically released when the referring WP_HTML_Token
	 * goes out of scope and is garbage-collected.
	 *
	 * @since 6.4.0
	 *
	 * @see WP_HTML_Processor::$release_internal_bookmark_on_destruct
	 *
	 * @var int
	 */
	private $bookmark_counter = 0;

	/**
	 * Stores an explanation for why something failed, if it did.
	 *
	 * @see self::get_last_error
	 *
	 * @since 6.4.0
	 *
	 * @var string|null
	 */
	private $last_error = 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
	 * the stack of open elements and the stack of active formatting elements without
	 * exposing it as a public method on the class.
	 *
	 * @since 6.4.0
	 *
	 * @var closure
	 */
	private $release_internal_bookmark_on_destruct = null;

	/*
	 * Public Interface Functions
	 */

	/**
	 * Creates an HTML processor in the fragment parsing mode.
	 *
	 * Use this for cases where you are processing chunks of HTML that
	 * will be found within a bigger HTML document, such as rendered
	 * block output that exists within a post, `the_content` inside a
	 * rendered site layout.
	 *
	 * Fragment parsing occurs within a context, which is an HTML element
	 * that the document will eventually be placed in. It becomes important
	 * when special elements have different rules than others, such as inside
	 * a TEXTAREA or a TITLE tag where things that look like tags are text,
	 * or inside a SCRIPT tag where things that look like HTML syntax are JS.
	 *
	 * The context value should be a representation of the tag into which the
	 * HTML is found. For most cases this will be the body element. The HTML
	 * form is provided because a context element may have attributes that
	 * impact the parse, such as with a SCRIPT tag and its `type` attribute.
	 *
	 * ## Current HTML Support
	 *
	 *  - The only supported context is `<body>`, which is the default value.
	 *  - The only supported document encoding is `UTF-8`, which is the default value.
	 *
	 * @since 6.4.0
	 *
	 * @param string $html     Input HTML fragment to process.
	 * @param string $context  Context element for the fragment, must be default of `<body>`.
	 * @param string $encoding Text encoding of the document; must be default of 'UTF-8'.
	 * @return WP_HTML_Processor|null The created processor if successful, otherwise null.
	 */
	public static function create_fragment( $html, $context = '<body>', $encoding = 'UTF-8' ) {
		if ( '<body>' !== $context || 'UTF-8' !== $encoding ) {
			return null;
		}

		$processor                        = new self( $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
			)
		);

		$processor->state->stack_of_open_elements->push(
			new WP_HTML_Token(
				'context-node',
				$processor->state->context_node[0],
				false
			)
		);

		return $processor;
	}

	/**
	 * Constructor.
	 *
	 * Do not use this method. Use the static creator methods instead.
	 *
	 * @access private
	 *
	 * @since 6.4.0
	 *
	 * @see WP_HTML_Processor::create_fragment()
	 *
	 * @param string      $html                                  HTML to process.
	 * @param string|null $use_the_static_create_methods_instead This constructor should not be called manually.
	 */
	public function __construct( $html, $use_the_static_create_methods_instead = null ) {
		parent::__construct( $html );

		if ( self::CONSTRUCTOR_UNLOCK_CODE !== $use_the_static_create_methods_instead ) {
			_doing_it_wrong(
				__METHOD__,
				sprintf(
					/* translators: %s: WP_HTML_Processor::create_fragment(). */
					__( 'Call %s to create an HTML Processor instead of calling the constructor directly.' ),
					'<code>WP_HTML_Processor::create_fragment()</code>'
				),
				'6.4.0'
			);
		}

		$this->state = new WP_HTML_Processor_State();

		/*
		 * Create this wrapper so that it's possible to pass
		 * a private method into WP_HTML_Token classes without
		 * exposing it to any public API.
		 */
		$this->release_internal_bookmark_on_destruct = function ( $name ) {
			parent::release_bookmark( $name );
		};
	}

	/**
	 * Returns the last error, if any.
	 *
	 * Various situations lead to parsing failure but this class will
	 * return `false` in all those cases. To determine why something
	 * failed it's possible to request the last error. This can be
	 * helpful to know to distinguish whether a given tag couldn't
	 * be found or if content in the document caused the processor
	 * to give up and abort processing.
	 *
	 * Example
	 *
	 *     $processor = WP_HTML_Processor::create_fragment( '<template><strong><button><em><p><em>' );
	 *     false === $processor->next_tag();
	 *     WP_HTML_Processor::ERROR_UNSUPPORTED === $processor->get_last_error();
	 *
	 * @since 6.4.0
	 *
	 * @see self::ERROR_UNSUPPORTED
	 * @see self::ERROR_EXCEEDED_MAX_BOOKMARKS
	 *
	 * @return string|null The last error, if one exists, otherwise null.
	 */
	public function get_last_error() {
		return $this->last_error;
	}

	/**
	 * Finds the next tag matching the $query.
	 *
	 * @todo Support matching the class name and tag name.
	 *
	 * @since 6.4.0
	 *
	 * @throws Exception When unable to allocate a bookmark for the next token in the input HTML document.
	 *
	 * @param array|string|null $query {
	 *     Optional. Which tag name to find, having which class, etc. Default is to find any tag.
	 *
	 *     @type string|null $tag_name     Which tag to find, or `null` for "any tag."
	 *     @type int|null    $match_offset Find the Nth tag matching all search criteria.
	 *                                     1 for "first" tag, 3 for "third," etc.
	 *                                     Defaults to first tag.
	 *     @type string|null $class_name   Tag must contain this whole class name to match.
	 *     @type string[]    $breadcrumbs  DOM sub-path at which element is found, e.g. `array( 'FIGURE', 'IMG' )`.
	 *                                     May also contain the wildcard `*` which matches a single element, e.g. `array( 'SECTION', '*' )`.
	 * }
	 * @return bool Whether a tag was matched.
	 */
	public function next_tag( $query = null ) {
		if ( null === $query ) {
			while ( $this->step() ) {
				if ( '#tag' !== $this->get_token_type() ) {
					continue;
				}

				if ( ! $this->is_tag_closer() ) {
					return true;
				}
			}

			return false;
		}

		if ( is_string( $query ) ) {
			$query = array( 'breadcrumbs' => array( $query ) );
		}

		if ( ! is_array( $query ) ) {
			_doing_it_wrong(
				__METHOD__,
				__( 'Please pass a query array to this function.' ),
				'6.4.0'
			);
			return false;
		}

		if ( ! ( array_key_exists( 'breadcrumbs', $query ) && is_array( $query['breadcrumbs'] ) ) ) {
			while ( $this->step() ) {
				if ( '#tag' !== $this->get_token_type() ) {
					continue;
				}

				if ( ! $this->is_tag_closer() ) {
					return true;
				}
			}

			return false;
		}

		if ( isset( $query['tag_closers'] ) && 'visit' === $query['tag_closers'] ) {
			_doing_it_wrong(
				__METHOD__,
				__( 'Cannot visit tag closers in HTML Processor.' ),
				'6.4.0'
			);
			return false;
		}

		$breadcrumbs  = $query['breadcrumbs'];
		$match_offset = isset( $query['match_offset'] ) ? (int) $query['match_offset'] : 1;

		while ( $match_offset > 0 && $this->step() ) {
			if ( '#tag' !== $this->get_token_type() ) {
				continue;
			}

			if ( $this->matches_breadcrumbs( $breadcrumbs ) && 0 === --$match_offset ) {
				return true;
			}
		}

		return false;
	}

	/**
	 * 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.
	 *
	 * @since 6.5.0 Added for internal support; do not use.
	 *
	 * @access private
	 *
	 * @return bool
	 */
	public function next_token() {
		return $this->step();
	}

	/**
	 * Indicates if the currently-matched tag matches the given breadcrumbs.
	 *
	 * A "*" represents a single tag wildcard, where any tag matches, but not no tags.
	 *
	 * At some point this function _may_ support a `**` syntax for matching any number
	 * of unspecified tags in the breadcrumb stack. This has been intentionally left
	 * out, however, to keep this function simple and to avoid introducing backtracking,
	 * which could open up surprising performance breakdowns.
	 *
	 * Example:
	 *
	 *     $processor = WP_HTML_Processor::create_fragment( '<div><span><figure><img></figure></span></div>' );
	 *     $processor->next_tag( 'img' );
	 *     true  === $processor->matches_breadcrumbs( array( 'figure', 'img' ) );
	 *     true  === $processor->matches_breadcrumbs( array( 'span', 'figure', 'img' ) );
	 *     false === $processor->matches_breadcrumbs( array( 'span', 'img' ) );
	 *     true  === $processor->matches_breadcrumbs( array( 'span', '*', 'img' ) );
	 *
	 * @since 6.4.0
	 *
	 * @param string[] $breadcrumbs DOM sub-path at which element is found, e.g. `array( 'FIGURE', 'IMG' )`.
	 *                              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 ) {
		// Everything matches when there are zero constraints.
		if ( 0 === count( $breadcrumbs ) ) {
			return true;
		}

		// Start at the last crumb.
		$crumb = end( $breadcrumbs );

		if ( '*' !== $crumb && $this->get_tag() !== strtoupper( $crumb ) ) {
			return false;
		}

		foreach ( $this->state->stack_of_open_elements->walk_up() as $node ) {
			$crumb = strtoupper( current( $breadcrumbs ) );

			if ( '*' !== $crumb && $node->node_name !== $crumb ) {
				return false;
			}

			if ( false === prev( $breadcrumbs ) ) {
				return true;
			}
		}

		return false;
	}

	/**
	 * Steps through the HTML document and stop at the next tag, if any.
	 *
	 * @since 6.4.0
	 *
	 * @throws Exception When unable to allocate a bookmark for the next token in the input HTML document.
	 *
	 * @see self::PROCESS_NEXT_NODE
	 * @see self::REPROCESS_CURRENT_NODE
	 *
	 * @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 ) {
		// Refuse to proceed if there was a previous error.
		if ( null !== $this->last_error ) {
			return false;
		}

		if ( self::REPROCESS_CURRENT_NODE !== $node_to_process ) {
			/*
			 * Void elements still hop onto the stack of open elements even though
			 * there's no corresponding closing tag. This is important for managing
			 * stack-based operations such as "navigate to parent node" or checking
			 * on an element's breadcrumbs.
			 *
			 * 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 (
				$top_node && (
					// Void elements.
					self::is_void( $top_node->node_name ) ||
					// Comments, text nodes, and other atomic tokens.
					'#' === $top_node->node_name[0] ||
					// Doctype declarations.
					'html' === $top_node->node_name
				)
			) {
				$this->state->stack_of_open_elements->pop();
			}
		}

		if ( self::PROCESS_NEXT_NODE === $node_to_process ) {
			parent::next_token();
		}

		// Finish stepping when there are no more tokens in the document.
		if (
			WP_HTML_Tag_Processor::STATE_INCOMPLETE_INPUT === $this->parser_state ||
			WP_HTML_Tag_Processor::STATE_COMPLETE === $this->parser_state
		) {
			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
		);

		try {
			switch ( $this->state->insertion_mode ) {
				case WP_HTML_Processor_State::INSERTION_MODE_IN_BODY:
					return $this->step_in_body();

				default:
					$this->last_error = self::ERROR_UNSUPPORTED;
					throw new WP_HTML_Unsupported_Exception( "No support for parsing in the '{$this->state->insertion_mode}' state." );
			}
		} catch ( WP_HTML_Unsupported_Exception $e ) {
			/*
			 * Exceptions are used in this class to escape deep call stacks that
			 * otherwise might involve messier calling and return conventions.
			 */
			return false;
		}
	}

	/**
	 * Computes the HTML breadcrumbs for the currently-matched node, if matched.
	 *
	 * 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
	 *
	 *     $processor = WP_HTML_Processor::create_fragment( '<p><strong><em><img></em></strong></p>' );
	 *     $processor->next_tag( 'IMG' );
	 *     $processor->get_breadcrumbs() === array( 'HTML', 'BODY', 'P', 'STRONG', 'EM', 'IMG' );
	 *
	 * @since 6.4.0
	 *
	 * @return string[]|null Array of tag names representing path to matched node, if matched, otherwise NULL.
	 */
	public function get_breadcrumbs() {
		$breadcrumbs = array();
		foreach ( $this->state->stack_of_open_elements->walk_down() as $stack_item ) {
			$breadcrumbs[] = $stack_item->node_name;
		}

		return $breadcrumbs;
	}

	/**
	 * Parses next element in the 'in body' insertion mode.
	 *
	 * This internal function performs the 'in body' insertion mode
	 * logic for the generalized WP_HTML_Processor::step() function.
	 *
	 * @since 6.4.0
	 *
	 * @throws WP_HTML_Unsupported_Exception When encountering unsupported HTML input.
	 *
	 * @see https://html.spec.whatwg.org/#parsing-main-inbody
	 * @see WP_HTML_Processor::step
	 *
	 * @return bool Whether an element was found.
	 */
	private function step_in_body() {
		$token_name = $this->get_token_name();
		$token_type = $this->get_token_type();
		$op_sigil   = '#tag' === $token_type ? ( $this->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
				 *
				 * Any successive sequence of NULL bytes is ignored and won't
				 * trigger active format reconstruction. Therefore, if the text
				 * only comprises NULL bytes then the token should be ignored
				 * 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
				) {
					// Parse error: ignore the token.
					return $this->step();
				}

				/*
				 * 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" ) ) {
					$this->state->frameset_ok = false;
				}

				$this->insert_html_element( $this->state->current_token );
				return true;

			case 'html':
				/*
				 * > A DOCTYPE token
				 * > Parse error. Ignore the token.
				 */
				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' );
				}

				$this->reconstruct_active_formatting_elements();
				$this->insert_html_element( $this->state->current_token );
				$this->state->frameset_ok = false;

				return true;

			/*
			 * > A start tag whose tag name is one of: "address", "article", "aside",
			 * > "blockquote", "center", "details", "dialog", "dir", "div", "dl",
			 * > "fieldset", "figcaption", "figure", "footer", "header", "hgroup",
			 * > "main", "menu", "nav", "ol", "p", "search", "section", "summary", "ul"
			 */
			case '+ADDRESS':
			case '+ARTICLE':
			case '+ASIDE':
			case '+BLOCKQUOTE':
			case '+CENTER':
			case '+DETAILS':
			case '+DIALOG':
			case '+DIR':
			case '+DIV':
			case '+DL':
			case '+FIELDSET':
			case '+FIGCAPTION':
			case '+FIGURE':
			case '+FOOTER':
			case '+HEADER':
			case '+HGROUP':
			case '+MAIN':
			case '+MENU':
			case '+NAV':
			case '+OL':
			case '+P':
			case '+SEARCH':
			case '+SECTION':
			case '+SUMMARY':
			case '+UL':
				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;

			/*
			 * > 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",
			 * > "menu", "nav", "ol", "pre", "search", "section", "summary", "ul"
			 */
			case '-ADDRESS':
			case '-ARTICLE':
			case '-ASIDE':
			case '-BLOCKQUOTE':
			case '-BUTTON':
			case '-CENTER':
			case '-DETAILS':
			case '-DIALOG':
			case '-DIR':
			case '-DIV':
			case '-DL':
			case '-FIELDSET':
			case '-FIGCAPTION':
			case '-FIGURE':
			case '-FOOTER':
			case '-HEADER':
			case '-HGROUP':
			case '-LISTING':
			case '-MAIN':
			case '-MENU':
			case '-NAV':
			case '-OL':
			case '-PRE':
			case '-SEARCH':
			case '-SECTION':
			case '-SUMMARY':
			case '-UL':
				if ( ! $this->state->stack_of_open_elements->has_element_in_scope( $token_name ) ) {
					// @todo Report parse error.
					// Ignore the token.
					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( $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)' ) ) {
					/*
					 * 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 !== $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 ) {
						// @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 &&
					$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;
				}

				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;

			/*
			 * > An end tag whose tag name is "li"
			 * > An end tag whose tag name is one of: "dd", "dt"
			 */
			case '-DD':
			case '-DT':
			case '-LI':
				if (
					/*
					 * An end tag whose tag name is "li":
					 * If the stack of open elements does not have an li element in list item scope,
					 * then this is a parse error; ignore the token.
					 */
					(
						'LI' === $token_name &&
						! $this->state->stack_of_open_elements->has_element_in_list_item_scope( 'LI' )
					) ||
					/*
					 * An end tag whose tag name is one of: "dd", "dt":
					 * If the stack of open elements does not have an element in scope that is an
					 * HTML element with the same tag name as that of the token, then this is a
					 * parse error; ignore the token.
					 */
					(
						'LI' !== $token_name &&
						! $this->state->stack_of_open_elements->has_element_in_scope( $token_name )
					)
				) {
					/*
					 * 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( $token_name );

				if ( $token_name !== $this->state->stack_of_open_elements->current_node()->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( $token_name );
				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 );
				}

				$this->close_a_p_element();
				return true;

			// > 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;

						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;
					}
				}

				$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;

			/*
			 * > A start tag whose tag name is one of: "b", "big", "code", "em", "font", "i",
			 * > "s", "small", "strike", "strong", "tt", "u"
			 */
			case '+B':
			case '+BIG':
			case '+CODE':
			case '+EM':
			case '+FONT':
			case '+I':
			case '+S':
			case '+SMALL':
			case '+STRIKE':
			case '+STRONG':
			case '+TT':
			case '+U':
				$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"
			 */
			case '-A':
			case '-B':
			case '-BIG':
			case '-CODE':
			case '-EM':
			case '-FONT':
			case '-I':
			case '-S':
			case '-SMALL':
			case '-STRIKE':
			case '-STRONG':
			case '-TT':
			case '-U':
				$this->run_adoption_agency_algorithm();
				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.' );

			/*
			 * > A start tag whose tag name is one of: "area", "br", "embed", "img", "keygen", "wbr"
			 */
			case '+AREA':
			case '+BR':
			case '+EMBED':
			case '+IMG':
			case '+KEYGEN':
			case '+WBR':
				$this->reconstruct_active_formatting_elements();
				$this->insert_html_element( $this->state->current_token );
				$this->state->frameset_ok = false;
				return true;

			/*
			 * > A start tag whose tag name is "input"
			 */
			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".
				 */
				if ( ! is_string( $type_attribute ) || 'hidden' !== strtolower( $type_attribute ) ) {
					$this->state->frameset_ok = false;
				}
				return true;

			/*
			 * > A start tag whose tag name is "hr"
			 */
			case '+HR':
				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;

			/*
			 * > 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;
		}

		/*
		 * 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." );
		}

		if ( ! $this->is_tag_closer() ) {
			/*
			 * > Any other start tag
			 */
			$this->reconstruct_active_formatting_elements();
			$this->insert_html_element( $this->state->current_token );
			return true;
		} else {
			/*
			 * > Any other end tag
			 */

			/*
			 * Find the corresponding tag opener in the stack of open elements, if
			 * it exists before reaching a special element, which provides a kind
			 * of boundary in the stack. For example, a `</custom-tag>` should not
			 * 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 ) {
					break;
				}

				if ( self::is_special( $node->node_name ) ) {
					// This is a parse error, ignore the token.
					return $this->step();
				}
			}

			$this->generate_implied_end_tags( $token_name );
			if ( $node !== $this->state->stack_of_open_elements->current_node() ) {
				// @todo Record parse error: this error doesn't impact parsing.
			}

			foreach ( $this->state->stack_of_open_elements->walk_up() as $item ) {
				$this->state->stack_of_open_elements->pop();
				if ( $node === $item ) {
					return true;
				}
			}
		}
	}

	/*
	 * Internal helpers
	 */

	/**
	 * Creates a new bookmark for the currently-matched token and returns the generated name.
	 *
	 * @since 6.4.0
	 * @since 6.5.0 Renamed from bookmark_tag() to bookmark_token().
	 *
	 * @throws Exception When unable to allocate requested bookmark.
	 *
	 * @return string|false Name of created bookmark, or false if unable to create.
	 */
	private function bookmark_token() {
		if ( ! parent::set_bookmark( ++$this->bookmark_counter ) ) {
			$this->last_error = self::ERROR_EXCEEDED_MAX_BOOKMARKS;
			throw new Exception( 'could not allocate bookmark' );
		}

		return "{$this->bookmark_counter}";
	}

	/*
	 * HTML semantic overrides for Tag Processor
	 */

	/**
	 * Returns the uppercase name of the matched tag.
	 *
	 * The semantic rules for HTML specify that certain tags be reprocessed
	 * with a different tag name. Because of this, the tag name presented
	 * by the HTML Processor may differ from the one reported by the HTML
	 * Tag Processor, which doesn't apply these semantic rules.
	 *
	 * Example:
	 *
	 *     $processor = new WP_HTML_Tag_Processor( '<div class="test">Test</div>' );
	 *     $processor->next_tag() === true;
	 *     $processor->get_tag() === 'DIV';
	 *
	 *     $processor->next_tag() === false;
	 *     $processor->get_tag() === null;
	 *
	 * @since 6.4.0
	 *
	 * @return string|null Name of currently matched tag in input HTML, or `null` if none found.
	 */
	public function get_tag() {
		if ( null !== $this->last_error ) {
			return null;
		}

		$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;
		}
	}

	/**
	 * Removes a bookmark that is no longer needed.
	 *
	 * Releasing a bookmark frees up the small
	 * performance overhead it requires.
	 *
	 * @since 6.4.0
	 *
	 * @param string $bookmark_name Name of the bookmark to remove.
	 * @return bool Whether the bookmark already existed before removal.
	 */
	public function release_bookmark( $bookmark_name ) {
		return parent::release_bookmark( "_{$bookmark_name}" );
	}

	/**
	 * Moves the internal cursor in the HTML Processor to a given bookmark's location.
	 *
	 * Be careful! Seeking backwards to a previous location resets the parser to the
	 * start of the document and reparses the entire contents up until it finds the
	 * sought-after bookmarked location.
	 *
	 * In order to prevent accidental infinite loops, there's a
	 * maximum limit on the number of times seek() can be called.
	 *
	 * @throws Exception When unable to allocate a bookmark for the next token in the input HTML document.
	 *
	 * @since 6.4.0
	 *
	 * @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 ) {
		// Flush any pending updates to the document before beginning.
		$this->get_updated_html();

		$actual_bookmark_name = "_{$bookmark_name}";
		$processor_started_at = $this->state->current_token
			? $this->bookmarks[ $this->state->current_token->bookmark_name ]->start
			: 0;
		$bookmark_starts_at   = $this->bookmarks[ $actual_bookmark_name ]->start;
		$direction            = $bookmark_starts_at > $processor_started_at ? 'forward' : 'backward';

		/*
		 * If seeking backwards, it's possible that the sought-after bookmark exists within an element
		 * which has been closed before the current cursor; in other words, it has already been removed
		 * from the stack of open elements. This means that it's insufficient to simply pop off elements
		 * from the stack of open elements which appear after the bookmarked location and then jump to
		 * that location, as the elements which were open before won't be re-opened.
		 *
		 * In order to maintain consistency, the HTML Processor rewinds to the start of the document
		 * and reparses everything until it finds the sought-after bookmark.
		 *
		 * There are potentially better ways to do this: cache the parser state for each bookmark and
		 * restore it when seeking; store an immutable and idempotent register of where elements open
		 * and close.
		 *
		 * If caching the parser state it will be essential to properly maintain the cached stack of
		 * open elements and active formatting elements when modifying the document. This could be a
		 * tedious and time-consuming process as well, and so for now will not be performed.
		 *
		 * It may be possible to track bookmarks for where elements open and close, and in doing so
		 * be able to quickly recalculate breadcrumbs for any element in the document. It may even
		 * be possible to remove the stack of open elements and compute it on the fly this way.
		 * If doing this, the parser would need to track the opening and closing locations for all
		 * tokens in the breadcrumb path for any and all bookmarks. By utilizing bookmarks themselves
		 * this list could be automatically maintained while modifying the document. Finding the
		 * breadcrumbs would then amount to traversing that list from the start until the token
		 * being inspected. Once an element closes, if there are no bookmarks pointing to locations
		 * within that element, then all of these locations may be forgotten to save on memory use
		 * 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.
			 */
			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;
		}

		// 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->step() ) {
			if ( $bookmark_starts_at === $this->bookmarks[ $this->state->current_token->bookmark_name ]->start ) {
				return true;
			}
		}

		return false;
	}

	/**
	 * Sets a bookmark in the HTML document.
	 *
	 * Bookmarks represent specific places or tokens in the HTML
	 * document, such as a tag opener or closer. When applying
	 * edits to a document, such as setting an attribute, the
	 * text offsets of that token may shift; the bookmark is
	 * kept updated with those shifts and remains stable unless
	 * the entire span of text in which the token sits is removed.
	 *
	 * Release bookmarks when they are no longer needed.
	 *
	 * Example:
	 *
	 *     <main><h2>Surprising fact you may not know!</h2></main>
	 *           ^  ^
	 *            \-|-- this `H2` opener bookmark tracks the token
	 *
	 *     <main class="clickbait"><h2>Surprising fact you may no…
	 *                             ^  ^
	 *                              \-|-- it shifts with edits
	 *
	 * Bookmarks provide the ability to seek to a previously-scanned
	 * place in the HTML document. This avoids the need to re-scan
	 * the entire document.
	 *
	 * Example:
	 *
	 *     <ul><li>One</li><li>Two</li><li>Three</li></ul>
	 *                                 ^^^^
	 *                                 want to note this last item
	 *
	 *     $p = new WP_HTML_Tag_Processor( $html );
	 *     $in_list = false;
	 *     while ( $p->next_tag( array( 'tag_closers' => $in_list ? 'visit' : 'skip' ) ) ) {
	 *         if ( 'UL' === $p->get_tag() ) {
	 *             if ( $p->is_tag_closer() ) {
	 *                 $in_list = false;
	 *                 $p->set_bookmark( 'resume' );
	 *                 if ( $p->seek( 'last-li' ) ) {
	 *                     $p->add_class( 'last-li' );
	 *                 }
	 *                 $p->seek( 'resume' );
	 *                 $p->release_bookmark( 'last-li' );
	 *                 $p->release_bookmark( 'resume' );
	 *             } else {
	 *                 $in_list = true;
	 *             }
	 *         }
	 *
	 *         if ( 'LI' === $p->get_tag() ) {
	 *             $p->set_bookmark( 'last-li' );
	 *         }
	 *     }
	 *
	 * Bookmarks intentionally hide the internal string offsets
	 * to which they refer. They are maintained internally as
	 * updates are applied to the HTML document and therefore
	 * retain their "position" - the location to which they
	 * originally pointed. The inability to use bookmarks with
	 * functions like `substr` is therefore intentional to guard
	 * against accidentally breaking the HTML.
	 *
	 * Because bookmarks allocate memory and require processing
	 * for every applied update, they are limited and require
	 * a name. They should not be created with programmatically-made
	 * names, such as "li_{$index}" with some loop. As a general
	 * rule they should only be created with string-literal names
	 * like "start-of-section" or "last-paragraph".
	 *
	 * Bookmarks are a powerful tool to enable complicated behavior.
	 * Consider double-checking that you need this tool if you are
	 * reaching for it, as inappropriate use could lead to broken
	 * HTML structure or unwanted processing overhead.
	 *
	 * @since 6.4.0
	 *
	 * @param string $bookmark_name Identifies this particular bookmark.
	 * @return bool Whether the bookmark was successfully created.
	 */
	public function set_bookmark( $bookmark_name ) {
		return parent::set_bookmark( "_{$bookmark_name}" );
	}

	/**
	 * Checks whether a bookmark with the given name exists.
	 *
	 * @since 6.5.0
	 *
	 * @param string $bookmark_name Name to identify a bookmark that potentially exists.
	 * @return bool Whether that bookmark exists.
	 */
	public function has_bookmark( $bookmark_name ) {
		return parent::has_bookmark( "_{$bookmark_name}" );
	}

	/*
	 * HTML Parsing Algorithms
	 */

	/**
	 * Closes a P element.
	 *
	 * @since 6.4.0
	 *
	 * @throws WP_HTML_Unsupported_Exception When encountering unsupported HTML input.
	 *
	 * @see https://html.spec.whatwg.org/#close-a-p-element
	 */
	private function close_a_p_element() {
		$this->generate_implied_end_tags( 'P' );
		$this->state->stack_of_open_elements->pop_until( 'P' );
	}

	/**
	 * Closes elements that have implied end tags.
	 *
	 * @since 6.4.0
	 *
	 * @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 ) {
		$elements_with_implied_end_tags = array(
			'DD',
			'DT',
			'LI',
			'P',
		);

		$current_node = $this->state->stack_of_open_elements->current_node();
		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 )
		) {
			$this->state->stack_of_open_elements->pop();
		}
	}

	/**
	 * Closes elements that have implied end tags, thoroughly.
	 *
	 * See the HTML specification for an explanation why this is
	 * different from generating end tags in the normal sense.
	 *
	 * @since 6.4.0
	 *
	 * @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() {
		$elements_with_implied_end_tags = array(
			'DD',
			'DT',
			'LI',
			'P',
		);

		while ( in_array( $this->state->stack_of_open_elements->current_node(), $elements_with_implied_end_tags, true ) ) {
			$this->state->stack_of_open_elements->pop();
		}
	}

	/**
	 * Reconstructs the active formatting elements.
	 *
	 * > This has the effect of reopening all the formatting elements that were opened
	 * > in the current body, cell, or caption (whichever is youngest) that haven't
	 * > been explicitly closed.
	 *
	 * @since 6.4.0
	 *
	 * @throws WP_HTML_Unsupported_Exception When encountering unsupported HTML input.
	 *
	 * @see https://html.spec.whatwg.org/#reconstruct-the-active-formatting-elements
	 *
	 * @return bool Whether any formatting elements needed to be reconstructed.
	 */
	private function reconstruct_active_formatting_elements() {
		/*
		 * > If there are no entries in the list of active formatting elements, then there is nothing
		 * > to reconstruct; stop this algorithm.
		 */
		if ( 0 === $this->state->active_formatting_elements->count() ) {
			return false;
		}

		$last_entry = $this->state->active_formatting_elements->current_node();
		if (

			/*
			 * > If the last (most recently added) entry in the list of active formatting elements is a marker;
			 * > stop this algorithm.
			 */
			'marker' === $last_entry->node_name ||

			/*
			 * > If the last (most recently added) entry in the list of active formatting elements is an
			 * > element that is in the stack of open elements, then there is nothing to reconstruct;
			 * > stop this algorithm.
			 */
			$this->state->stack_of_open_elements->contains_node( $last_entry )
		) {
			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.' );
	}

	/**
	 * Runs the adoption agency algorithm.
	 *
	 * @since 6.4.0
	 *
	 * @throws WP_HTML_Unsupported_Exception When encountering unsupported HTML input.
	 *
	 * @see https://html.spec.whatwg.org/#adoption-agency-algorithm
	 */
	private function run_adoption_agency_algorithm() {
		$budget       = 1000;
		$subject      = $this->get_tag();
		$current_node = $this->state->stack_of_open_elements->current_node();

		if (
			// > If the current node is an HTML element whose tag name is subject
			$current_node && $subject === $current_node->node_name &&
			// > the current node is not in the list of active formatting elements
			! $this->state->active_formatting_elements->contains_node( $current_node )
		) {
			$this->state->stack_of_open_elements->pop();
			return;
		}

		$outer_loop_counter = 0;
		while ( $budget-- > 0 ) {
			if ( $outer_loop_counter++ >= 8 ) {
				return;
			}

			/*
			 * > Let formatting element be the last element in the list of active formatting elements that:
			 * >   - is between the end of the list and the last marker in the list,
			 * >     if any, or the start of the list otherwise,
			 * >   - and has the tag name subject.
			 */
			$formatting_element = null;
			foreach ( $this->state->active_formatting_elements->walk_up() as $item ) {
				if ( 'marker' === $item->node_name ) {
					break;
				}

				if ( $subject === $item->node_name ) {
					$formatting_element = $item;
					break;
				}
			}

			// > 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.' );
			}

			// > 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.
			if ( ! $this->state->stack_of_open_elements->contains_node( $formatting_element ) ) {
				$this->state->active_formatting_elements->remove_node( $formatting_element );
				return;
			}

			// > If formatting element is in the stack of open elements, but the element is not in scope, then this is a parse error; return.
			if ( ! $this->state->stack_of_open_elements->has_element_in_scope( $formatting_element->node_name ) ) {
				return;
			}

			/*
			 * > Let furthest block be the topmost node in the stack of open elements that is lower in the stack
			 * > than formatting element, and is an element in the special category. There might not be one.
			 */
			$is_above_formatting_element = true;
			$furthest_block              = null;
			foreach ( $this->state->stack_of_open_elements->walk_down() as $item ) {
				if ( $is_above_formatting_element && $formatting_element->bookmark_name !== $item->bookmark_name ) {
					continue;
				}

				if ( $is_above_formatting_element ) {
					$is_above_formatting_element = false;
					continue;
				}

				if ( self::is_special( $item->node_name ) ) {
					$furthest_block = $item;
					break;
				}
			}

			/*
			 * > If there is no furthest block, then the UA must first pop all the nodes from the bottom of the
			 * > stack of open elements, from the current node up to and including formatting element, then
			 * > remove formatting element from the list of active formatting elements, and finally return.
			 */
			if ( null === $furthest_block ) {
				foreach ( $this->state->stack_of_open_elements->walk_up() as $item ) {
					$this->state->stack_of_open_elements->pop();

					if ( $formatting_element->bookmark_name === $item->bookmark_name ) {
						$this->state->active_formatting_elements->remove_node( $formatting_element );
						return;
					}
				}
			}

			$this->last_error = self::ERROR_UNSUPPORTED;
			throw new WP_HTML_Unsupported_Exception( '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.' );
	}

	/**
	 * Inserts an HTML element on the stack of open elements.
	 *
	 * @since 6.4.0
	 *
	 * @see https://html.spec.whatwg.org/#insert-a-foreign-element
	 *
	 * @param WP_HTML_Token $token Name of bookmark pointing to element in original input HTML.
	 */
	private function insert_html_element( $token ) {
		$this->state->stack_of_open_elements->push( $token );
	}

	/*
	 * HTML Specification Helpers
	 */

	/**
	 * 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.
	 * @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 );

		return (
			'ADDRESS' === $tag_name ||
			'APPLET' === $tag_name ||
			'AREA' === $tag_name ||
			'ARTICLE' === $tag_name ||
			'ASIDE' === $tag_name ||
			'BASE' === $tag_name ||
			'BASEFONT' === $tag_name ||
			'BGSOUND' === $tag_name ||
			'BLOCKQUOTE' === $tag_name ||
			'BODY' === $tag_name ||
			'BR' === $tag_name ||
			'BUTTON' === $tag_name ||
			'CAPTION' === $tag_name ||
			'CENTER' === $tag_name ||
			'COL' === $tag_name ||
			'COLGROUP' === $tag_name ||
			'DD' === $tag_name ||
			'DETAILS' === $tag_name ||
			'DIR' === $tag_name ||
			'DIV' === $tag_name ||
			'DL' === $tag_name ||
			'DT' === $tag_name ||
			'EMBED' === $tag_name ||
			'FIELDSET' === $tag_name ||
			'FIGCAPTION' === $tag_name ||
			'FIGURE' === $tag_name ||
			'FOOTER' === $tag_name ||
			'FORM' === $tag_name ||
			'FRAME' === $tag_name ||
			'FRAMESET' === $tag_name ||
			'H1' === $tag_name ||
			'H2' === $tag_name ||
			'H3' === $tag_name ||
			'H4' === $tag_name ||
			'H5' === $tag_name ||
			'H6' === $tag_name ||
			'HEAD' === $tag_name ||
			'HEADER' === $tag_name ||
			'HGROUP' === $tag_name ||
			'HR' === $tag_name ||
			'HTML' === $tag_name ||
			'IFRAME' === $tag_name ||
			'IMG' === $tag_name ||
			'INPUT' === $tag_name ||
			'KEYGEN' === $tag_name ||
			'LI' === $tag_name ||
			'LINK' === $tag_name ||
			'LISTING' === $tag_name ||
			'MAIN' === $tag_name ||
			'MARQUEE' === $tag_name ||
			'MENU' === $tag_name ||
			'META' === $tag_name ||
			'NAV' === $tag_name ||
			'NOEMBED' === $tag_name ||
			'NOFRAMES' === $tag_name ||
			'NOSCRIPT' === $tag_name ||
			'OBJECT' === $tag_name ||
			'OL' === $tag_name ||
			'P' === $tag_name ||
			'PARAM' === $tag_name ||
			'PLAINTEXT' === $tag_name ||
			'PRE' === $tag_name ||
			'SCRIPT' === $tag_name ||
			'SEARCH' === $tag_name ||
			'SECTION' === $tag_name ||
			'SELECT' === $tag_name ||
			'SOURCE' === $tag_name ||
			'STYLE' === $tag_name ||
			'SUMMARY' === $tag_name ||
			'TABLE' === $tag_name ||
			'TBODY' === $tag_name ||
			'TD' === $tag_name ||
			'TEMPLATE' === $tag_name ||
			'TEXTAREA' === $tag_name ||
			'TFOOT' === $tag_name ||
			'TH' === $tag_name ||
			'THEAD' === $tag_name ||
			'TITLE' === $tag_name ||
			'TR' === $tag_name ||
			'TRACK' === $tag_name ||
			'UL' === $tag_name ||
			'WBR' === $tag_name ||
			'XMP' === $tag_name ||

			// MathML.
			'MI' === $tag_name ||
			'MO' === $tag_name ||
			'MN' === $tag_name ||
			'MS' === $tag_name ||
			'MTEXT' === $tag_name ||
			'ANNOTATION-XML' === $tag_name ||

			// SVG.
			'FOREIGNOBJECT' === $tag_name ||
			'DESC' === $tag_name ||
			'TITLE' === $tag_name
		);
	}

	/**
	 * Returns whether a given element is an HTML Void Element
	 *
	 * > area, base, br, col, embed, hr, img, input, link, meta, source, track, wbr
	 *
	 * @since 6.4.0
	 *
	 * @see https://html.spec.whatwg.org/#void-elements
	 *
	 * @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 ) {
		$tag_name = strtoupper( $tag_name );

		return (
			'AREA' === $tag_name ||
			'BASE' === $tag_name ||
			'BASEFONT' === $tag_name || // Obsolete but still treated as void.
			'BGSOUND' === $tag_name || // Obsolete but still treated as void.
			'BR' === $tag_name ||
			'COL' === $tag_name ||
			'EMBED' === $tag_name ||
			'FRAME' === $tag_name ||
			'HR' === $tag_name ||
			'IMG' === $tag_name ||
			'INPUT' === $tag_name ||
			'KEYGEN' === $tag_name || // Obsolete but still treated as void.
			'LINK' === $tag_name ||
			'META' === $tag_name ||
			'PARAM' === $tag_name || // Obsolete but still treated as void.
			'SOURCE' === $tag_name ||
			'TRACK' === $tag_name ||
			'WBR' === $tag_name
		);
	}

	/*
	 * Constants that would pollute the top of the class if they were found there.
	 */

	/**
	 * Indicates that the next HTML token should be parsed and processed.
	 *
	 * @since 6.4.0
	 *
	 * @var string
	 */
	const PROCESS_NEXT_NODE = 'process-next-node';

	/**
	 * Indicates that the current HTML token should be reprocessed in the newly-selected insertion mode.
	 *
	 * @since 6.4.0
	 *
	 * @var string
	 */
	const REPROCESS_CURRENT_NODE = 'reprocess-current-node';

	/**
	 * Indicates that the current HTML token should be processed without advancing the parser.
	 *
	 * @since 6.5.0
	 *
	 * @var string
	 */
	const PROCESS_CURRENT_NODE = 'process-current-node';

	/**
	 * Indicates that the parser encountered unsupported markup and has bailed.
	 *
	 * @since 6.4.0
	 *
	 * @var string
	 */
	const ERROR_UNSUPPORTED = 'unsupported';

	/**
	 * Indicates that the parser encountered more HTML tokens than it
	 * was able to process and has bailed.
	 *
	 * @since 6.4.0
	 *
	 * @var string
	 */
	const ERROR_EXCEEDED_MAX_BOOKMARKS = 'exceeded-max-bookmarks';

	/**
	 * Unlock code that must be passed into the constructor to create this class.
	 *
	 * This class extends the WP_HTML_Tag_Processor, which has a public class
	 * constructor. Therefore, it's not possible to have a private constructor here.
	 *
	 * This unlock code is used to ensure that anyone calling the constructor is
	 * doing so with a full understanding that it's intended to be a private API.
	 *
	 * @access private
	 */
	const CONSTRUCTOR_UNLOCK_CODE = 'Use WP_HTML_Processor::create_fragment() instead of calling the class constructor directly.';
}

Changelog

VersionDescription
6.4.0Introduced.

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