wikimedia / mediawiki

includes/tidy/Balancer.php

<?php
/**
 * An implementation of the tree building portion of the HTML5 parsing
 * spec.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License along
 * with this program; if not, write to the Free Software Foundation, Inc.,
 * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 * http://www.gnu.org/copyleft/gpl.html
 *
 * @file
 * @ingroup Parser
 * @since 1.27
 * @author C. Scott Ananian, 2016
 */
namespace MediaWiki\Tidy;

use Wikimedia\Assert\Assert;
use Wikimedia\Assert\ParameterAssertionException;
use \ExplodeIterator;
use \IteratorAggregate;
use \ReverseArrayIterator;
use \Sanitizer;

// A note for future librarization[1] -- this file is a good candidate
// for splitting into an independent library, except that it is currently
// highly optimized for MediaWiki use.  It only implements the portions
// of the HTML5 tree builder used by tags supported by MediaWiki, and
// does not contain a true tokenizer pass, instead relying on
// comment stripping, attribute normalization, and escaping done by
// the MediaWiki Sanitizer.  It also deliberately avoids building
// a true DOM in memory, instead serializing elements to an output string
// as soon as possible (usually as soon as the tag is closed) to reduce
// its memory footprint.

// We've been gradually lifting some of these restrictions to handle
// non-sanitized output generated by extensions, but we shortcut the tokenizer
// for speed (primarily by splitting on `<`) and so rely on syntactic
// well-formedness.

// On the other hand, I've been pretty careful to note with comments in the
// code the places where this implementation omits features of the spec or
// depends on the MediaWiki Sanitizer.  Perhaps in the future we'll want to
// implement the missing pieces and make this a standalone PHP HTML5 parser.
// In order to do so, some sort of MediaWiki-specific API will need
// to be added to (a) allow the Balancer to bypass the tokenizer,
// and (b) support on-the-fly flattening instead of DOM node creation.

// [1]: https://www.mediawiki.org/wiki/Library_infrastructure_for_MediaWiki

/**
 * Utility constants and sets for the HTML5 tree building algorithm.
 * Sets are associative arrays indexed first by namespace and then by
 * lower-cased tag name.
 *
 * @ingroup Parser
 * @since 1.27
 */
class BalanceSets {
	const HTML_NAMESPACE = 'http://www.w3.org/1999/xhtml';
	const MATHML_NAMESPACE = 'http://www.w3.org/1998/Math/MathML';
	const SVG_NAMESPACE = 'http://www.w3.org/2000/svg';

	public static $unsupportedSet = [
		self::HTML_NAMESPACE => [
			'html' => true, 'head' => true, 'body' => true, 'frameset' => true,
			'frame' => true,
			'plaintext' => true, 'isindex' => true,
			'xmp' => true, 'iframe' => true, 'noembed' => true,
			'noscript' => true, 'script' => true,
			'title' => true
		]
	];

	public static $emptyElementSet = [
		self::HTML_NAMESPACE => [
			'area' => true, 'base' => true, 'basefont' => true,
			'bgsound' => true, 'br' => true, 'col' => true, 'command' => true,
			'embed' => true, 'frame' => true, 'hr' => true, 'img' => true,
			'input' => true, 'keygen' => true, 'link' => true, 'meta' => true,
			'param' => true, 'source' => true, 'track' => true, 'wbr' => true
		]
	];

	public static $extraLinefeedSet = [
		self::HTML_NAMESPACE => [
			'pre' => true, 'textarea' => true, 'listing' => true,
		]
	];

	public static $headingSet = [
		self::HTML_NAMESPACE => [
			'h1' => true, 'h2' => true, 'h3' => true,
			'h4' => true, 'h5' => true, 'h6' => true
		]
	];

	public static $specialSet = [
		self::HTML_NAMESPACE => [
			'address' => true, 'applet' => true, 'area' => true,
			'article' => true, 'aside' => true, 'base' => true,
			'basefont' => true, 'bgsound' => true, 'blockquote' => true,
			'body' => true, 'br' => true, 'button' => true, 'caption' => true,
			'center' => true, 'col' => true, 'colgroup' => true, 'dd' => true,
			'details' => true, 'dir' => true, 'div' => true, 'dl' => true,
			'dt' => true, 'embed' => true, 'fieldset' => true,
			'figcaption' => true, 'figure' => true, 'footer' => true,
			'form' => true, 'frame' => true, 'frameset' => true, 'h1' => true,
			'h2' => true, 'h3' => true, 'h4' => true, 'h5' => true,
			'h6' => true, 'head' => true, 'header' => true, 'hgroup' => true,
			'hr' => true, 'html' => true, 'iframe' => true, 'img' => true,
			'input' => true, 'isindex' => true, 'li' => true, 'link' => true,
			'listing' => true, 'main' => true, 'marquee' => true,
			'menu' => true, 'menuitem' => true, 'meta' => true, 'nav' => true,
			'noembed' => true, 'noframes' => true, 'noscript' => true,
			'object' => true, 'ol' => true, 'p' => true, 'param' => true,
			'plaintext' => true, 'pre' => true, 'script' => true,
			'section' => true, 'select' => true, 'source' => true,
			'style' => true, 'summary' => true, 'table' => true,
			'tbody' => true, 'td' => true, 'template' => true,
			'textarea' => true, 'tfoot' => true, 'th' => true, 'thead' => true,
			'title' => true, 'tr' => true, 'track' => true, 'ul' => true,
			'wbr' => true, 'xmp' => true
		],
		self::SVG_NAMESPACE => [
			'foreignobject' => true, 'desc' => true, 'title' => true
		],
		self::MATHML_NAMESPACE => [
			'mi' => true, 'mo' => true, 'mn' => true, 'ms' => true,
			'mtext' => true, 'annotation-xml' => true
		]
	];

	public static $addressDivPSet = [
		self::HTML_NAMESPACE => [
			'address' => true, 'div' => true, 'p' => true
		]
	];

	public static $tableSectionRowSet = [
		self::HTML_NAMESPACE => [
			'table' => true, 'thead' => true, 'tbody' => true,
			'tfoot' => true, 'tr' => true
		]
	];

	public static $impliedEndTagsSet = [
		self::HTML_NAMESPACE => [
			'dd' => true, 'dt' => true, 'li' => true, 'optgroup' => true,
			'option' => true, 'p' => true, 'rb' => true, 'rp' => true,
			'rt' => true, 'rtc' => true
		]
	];

	public static $thoroughImpliedEndTagsSet = [
		self::HTML_NAMESPACE => [
			'caption' => true, 'colgroup' => true, 'dd' => true, 'dt' => true,
			'li' => true, 'optgroup' => true, 'option' => true, 'p' => true,
			'rb' => true, 'rp' => true, 'rt' => true, 'rtc' => true,
			'tbody' => true, 'td' => true, 'tfoot' => true, 'th' => true,
			'thead' => true, 'tr' => true
		]
	];

	public static $tableCellSet = [
		self::HTML_NAMESPACE => [
			'td' => true, 'th' => true
		]
	];
	public static $tableContextSet = [
		self::HTML_NAMESPACE => [
			'table' => true, 'template' => true, 'html' => true
		]
	];

	public static $tableBodyContextSet = [
		self::HTML_NAMESPACE => [
			'tbody' => true, 'tfoot' => true, 'thead' => true,
			'template' => true, 'html' => true
		]
	];

	public static $tableRowContextSet = [
		self::HTML_NAMESPACE => [
			'tr' => true, 'template' => true, 'html' => true
		]
	];

	// See https://html.spec.whatwg.org/multipage/forms.html#form-associated-element
	public static $formAssociatedSet = [
		self::HTML_NAMESPACE => [
			'button' => true, 'fieldset' => true, 'input' => true,
			'keygen' => true, 'object' => true, 'output' => true,
			'select' => true, 'textarea' => true, 'img' => true
		]
	];

	public static $inScopeSet = [
		self::HTML_NAMESPACE => [
			'applet' => true, 'caption' => true, 'html' => true,
			'marquee' => true, 'object' => true,
			'table' => true, 'td' => true, 'template' => true,
			'th' => true
		],
		self::SVG_NAMESPACE => [
			'foreignobject' => true, 'desc' => true, 'title' => true
		],
		self::MATHML_NAMESPACE => [
			'mi' => true, 'mo' => true, 'mn' => true, 'ms' => true,
			'mtext' => true, 'annotation-xml' => true
		]
	];

	private static $inListItemScopeSet = null;
	public static function inListItemScopeSet() {
		if ( self::$inListItemScopeSet === null ) {
			self::$inListItemScopeSet = self::$inScopeSet;
			self::$inListItemScopeSet[self::HTML_NAMESPACE]['ol'] = true;
			self::$inListItemScopeSet[self::HTML_NAMESPACE]['ul'] = true;
		}
		return self::$inListItemScopeSet;
	}

	private static $inButtonScopeSet = null;
	public static function inButtonScopeSet() {
		if ( self::$inButtonScopeSet === null ) {
			self::$inButtonScopeSet = self::$inScopeSet;
			self::$inButtonScopeSet[self::HTML_NAMESPACE]['button'] = true;
		}
		return self::$inButtonScopeSet;
	}

	public static $inTableScopeSet = [
		self::HTML_NAMESPACE => [
			'html' => true, 'table' => true, 'template' => true
		]
	];

	public static $inInvertedSelectScopeSet = [
		self::HTML_NAMESPACE => [
			'option' => true, 'optgroup' => true
		]
	];

	public static $mathmlTextIntegrationPointSet = [
		self::MATHML_NAMESPACE => [
			'mi' => true, 'mo' => true, 'mn' => true, 'ms' => true,
			'mtext' => true
		]
	];

	public static $htmlIntegrationPointSet = [
		self::SVG_NAMESPACE => [
			'foreignobject' => true,
			'desc' => true,
			'title' => true
		]
	];

	// For tidy compatibility.
	public static $tidyPWrapSet = [
		self::HTML_NAMESPACE => [
			'body' => true, 'blockquote' => true,
			// We parse with <body> as the fragment context, but the top-level
			// element on the stack is actually <html>.  We could use the
			// "adjusted current node" everywhere to work around this, but it's
			// easier just to add <html> to the p-wrap set.
			'html' => true,
		],
	];
	public static $tidyInlineSet = [
		self::HTML_NAMESPACE => [
			'a' => true, 'abbr' => true, 'acronym' => true, 'applet' => true,
			'b' => true, 'basefont' => true, 'bdo' => true, 'big' => true,
			'br' => true, 'button' => true, 'cite' => true, 'code' => true,
			'dfn' => true, 'em' => true, 'font' => true, 'i' => true,
			'iframe' => true, 'img' => true, 'input' => true, 'kbd' => true,
			'label' => true, 'legend' => true, 'map' => true, 'object' => true,
			'param' => true, 'q' => true, 'rb' => true, 'rbc' => true,
			'rp' => true, 'rt' => true, 'rtc' => true, 'ruby' => true,
			's' => true, 'samp' => true, 'select' => true, 'small' => true,
			'span' => true, 'strike' => true, 'strong' => true, 'sub' => true,
			'sup' => true, 'textarea' => true, 'tt' => true, 'u' => true,
			'var' => true,
		],
	];
}

/**
 * A BalanceElement is a simplified version of a DOM Node.  The main
 * difference is that we only keep BalanceElements around for nodes
 * currently on the BalanceStack of open elements.  As soon as an
 * element is closed, with some minor exceptions relating to the
 * tree builder "adoption agency algorithm", the element and all its
 * children are serialized to a string using the flatten() method.
 * This keeps our memory usage low.
 *
 * @ingroup Parser
 * @since 1.27
 */
class BalanceElement {
	/**
	 * The namespace of the element.
	 * @var string $namespaceURI
	 */
	public $namespaceURI;
	/**
	 * The lower-cased name of the element.
	 * @var string $localName
	 */
	public $localName;
	/**
	 * Attributes for the element, in array form
	 * @var array $attribs
	 */
	public $attribs;

	/**
	 * Parent of this element, or the string "flat" if this element has
	 * already been flattened into its parent.
	 * @var BalanceElement|string|null $parent
	 */
	public $parent;

	/**
	 * An array of children of this element.  Typically only the last
	 * child will be an actual BalanceElement object; the rest will
	 * be strings, representing either text nodes or flattened
	 * BalanceElement objects.
	 * @var BalanceElement[]|string[] $children
	 */
	public $children;

	/**
	 * A unique string identifier for Noah's Ark purposes, lazy initialized
	 */
	private $noahKey;

	/**
	 * The next active formatting element in the list, or null if this is the
	 * end of the AFE list or if the element is not in the AFE list.
	 */
	public $nextAFE;

	/**
	 * The previous active formatting element in the list, or null if this is
	 * the start of the list or if the element is not in the AFE list.
	 */
	public $prevAFE;

	/**
	 * The next element in the Noah's Ark species bucket.
	 */
	public $nextNoah;

	/**
	 * Make a new BalanceElement corresponding to the HTML DOM Element
	 * with the given localname, namespace, and attributes.
	 *
	 * @param string $namespaceURI The namespace of the element.
	 * @param string $localName The lowercased name of the tag.
	 * @param array $attribs Attributes of the element
	 */
	public function __construct( $namespaceURI, $localName, array $attribs ) {
		$this->localName = $localName;
		$this->namespaceURI = $namespaceURI;
		$this->attribs = $attribs;
		$this->contents = '';

The property contents does not exist. Did you maybe forget to declare it?

		$this->parent = null;
		$this->children = [];
	}

	/**
	 * Remove the given child from this element.
	 * @param BalanceElement $elt
	 */
	private function removeChild( BalanceElement $elt ) {
		Assert::precondition(
			$this->parent !== 'flat', "Can't removeChild after flattening $this"
		);
		Assert::parameter(
			$elt->parent === $this, 'elt', 'must have $this as a parent'
		);
		$idx = array_search( $elt, $this->children, true );
		Assert::parameter( $idx !== false, '$elt', 'must be a child of $this' );
		$elt->parent = null;
		array_splice( $this->children, $idx, 1 );
	}

	/**
	 * Find $a in the list of children and insert $b before it.
	 * @param BalanceElement $a
	 * @param BalanceElement|string $b
	 */
	public function insertBefore( BalanceElement $a, $b ) {
		Assert::precondition(
			$this->parent !== 'flat', "Can't insertBefore after flattening."
		);
		$idx = array_search( $a, $this->children, true );
		Assert::parameter( $idx !== false, '$a', 'must be a child of $this' );
		if ( is_string( $b ) ) {
			array_splice( $this->children, $idx, 0, [ $b ] );
		} else {
			Assert::parameter( $b->parent !== 'flat', '$b', "Can't be flat" );
			if ( $b->parent !== null ) {
				$b->parent->removeChild( $b );
			}
			array_splice( $this->children, $idx, 0, [ $b ] );
			$b->parent = $this;
		}
	}

	/**
	 * Append $elt to the end of the list of children.
	 * @param BalanceElement|string $elt
	 */
	public function appendChild( $elt ) {
		Assert::precondition(
			$this->parent !== 'flat', "Can't appendChild after flattening."
		);
		if ( is_string( $elt ) ) {
			array_push( $this->children, $elt );
			return;
		}
		// Remove $elt from parent, if it had one.
		if ( $elt->parent !== null ) {
			$elt->parent->removeChild( $elt );
		}
		array_push( $this->children, $elt );
		$elt->parent = $this;
	}

	/**
	 * Transfer all of the children of $elt to $this.
	 * @param BalanceElement $elt
	 */
	public function adoptChildren( BalanceElement $elt ) {
		Assert::precondition(
			$elt->parent !== 'flat', "Can't adoptChildren after flattening."
		);
		foreach ( $elt->children as $child ) {
			if ( !is_string( $child ) ) {
				// This is an optimization which avoids an O(n^2) set of
				// array_splice operations.
				$child->parent = null;
			}
			$this->appendChild( $child );
		}
		$elt->children = [];
	}

	/**
	 * Flatten this node and all of its children into a string, as specified
	 * by the HTML serialization specification, and replace this node
	 * in its parent by that string.
	 *
	 * @param array $config Balancer configuration; see Balancer::__construct().
	 * @return string
	 *
	 * @see __toString()
	 */
	public function flatten( array $config ) {
		Assert::parameter( $this->parent !== null, '$this', 'must be a child' );
		Assert::parameter( $this->parent !== 'flat', '$this', 'already flat' );
		$idx = array_search( $this, $this->parent->children, true );
		Assert::parameter(
			$idx !== false, '$this', 'must be a child of its parent'
		);
		$tidyCompat = $config['tidyCompat'];
		if ( $tidyCompat ) {
			$blank = true;
			foreach ( $this->children as $elt ) {
				if ( !is_string( $elt ) ) {
					$elt = $elt->flatten( $config );
				}
				if ( $blank && preg_match( '/[^\t\n\f\r ]/', $elt ) ) {
					$blank = false;
				}
			}
			if ( $this->isHtmlNamed( 'mw:p-wrap' ) ) {
				$this->localName = 'p';
			} elseif ( $blank ) {
				// Add 'mw-empty-elt' class so elements can be hidden via CSS
				// for compatibility with legacy tidy.
				if ( !count( $this->attribs ) &&
					( $this->localName === 'tr' || $this->localName === 'li' )
				) {
					$this->attribs = [ 'class' => "mw-empty-elt" ];
				}
				$blank = false;
			}
			$flat = $blank ? '' : "{$this}";
		} else {
			$flat = "{$this}";
		}
		$this->parent->children[$idx] = $flat;
		$this->parent = 'flat'; // for assertion checking
		return $flat;
	}

	/**
	 * Serialize this node and all of its children to a string, as specified
	 * by the HTML serialization specification.
	 *
	 * @return string The serialization of the BalanceElement
	 * @see https://html.spec.whatwg.org/multipage/syntax.html#serialising-html-fragments
	 */
	public function __toString() {
		$encAttribs = '';
		foreach ( $this->attribs as $name => $value ) {
			$encValue = Sanitizer::encodeAttribute( $value );
			$encAttribs .= " $name=\"$encValue\"";
		}
		if ( !$this->isA( BalanceSets::$emptyElementSet ) ) {
			$out = "<{$this->localName}{$encAttribs}>";
			$len = strlen( $out );
			// flatten children
			foreach ( $this->children as $elt ) {
				$out .= "{$elt}";
			}
			$out .= "</{$this->localName}>";
			if (
				$this->isA( BalanceSets::$extraLinefeedSet ) &&
				$out[$len] === "\n"
			) {
				// Double the linefeed after pre/listing/textarea
				// according to the HTML5 fragment serialization algorithm.
				$out = substr( $out, 0, $len + 1 ) .
					substr( $out, $len );
			}
		} else {
			$out = "<{$this->localName}{$encAttribs} />";
			Assert::invariant(
				count( $this->children ) === 0,
				"Empty elements shouldn't have children."
			);
		}
		return $out;
	}

	// Utility functions on BalanceElements.

	/**
	 * Determine if $this represents a specific HTML tag, is a member of
	 * a tag set, or is equal to another BalanceElement.
	 *
	 * @param BalanceElement|array|string $set The target BalanceElement,
	 *   set (from the BalanceSets class), or string (HTML tag name).
	 * @return bool
	 */
	public function isA( $set ) {
		if ( $set instanceof BalanceElement ) {
			return $this === $set;
		} elseif ( is_array( $set ) ) {
			return isset( $set[$this->namespaceURI] ) &&
				isset( $set[$this->namespaceURI][$this->localName] );
		} else {
			// assume this is an HTML element name.
			return $this->isHtml() && $this->localName === $set;
		}
	}

	/**
	 * Determine if this element is an HTML element with the specified name
	 * @param string $tagName
	 * @return bool
	 */
	public function isHtmlNamed( $tagName ) {
		return $this->namespaceURI === BalanceSets::HTML_NAMESPACE
			&& $this->localName === $tagName;
	}

	/**
	 * Determine if $this represents an element in the HTML namespace.
	 *
	 * @return bool
	 */
	public function isHtml() {
		return $this->namespaceURI === BalanceSets::HTML_NAMESPACE;
	}

	/**
	 * Determine if $this represents a MathML text integration point,
	 * as defined in the HTML5 specification.
	 *
	 * @return bool
	 * @see https://html.spec.whatwg.org/multipage/syntax.html#mathml-text-integration-point
	 */
	public function isMathmlTextIntegrationPoint() {
		return $this->isA( BalanceSets::$mathmlTextIntegrationPointSet );
	}

	/**
	 * Determine if $this represents an HTML integration point,
	 * as defined in the HTML5 specification.
	 *
	 * @return bool
	 * @see https://html.spec.whatwg.org/multipage/syntax.html#html-integration-point
	 */
	public function isHtmlIntegrationPoint() {
		if ( $this->isA( BalanceSets::$htmlIntegrationPointSet ) ) {
			return true;
		}
		if (
			$this->namespaceURI === BalanceSets::MATHML_NAMESPACE &&
			$this->localName === 'annotation-xml' &&
			isset( $this->attribs['encoding'] ) &&
			( strcasecmp( $this->attribs['encoding'], 'text/html' ) == 0 ||
			strcasecmp( $this->attribs['encoding'], 'application/xhtml+xml' ) == 0 )
		) {
			return true;
		}
		return false;
	}

	/**
	 * Get a string key for the Noah's Ark algorithm
	 */
	public function getNoahKey() {
		if ( $this->noahKey === null ) {
			$attribs = $this->attribs;
			ksort( $attribs );
			$this->noahKey = serialize( [ $this->namespaceURI, $this->localName, $attribs ] );
		}
		return $this->noahKey;
	}
}

/**
 * The "stack of open elements" as defined in the HTML5 tree builder
 * spec.  This contains methods to ensure that content (start tags, text)
 * are inserted at the correct place in the output string, and to
 * flatten BalanceElements are they are closed to avoid holding onto
 * a complete DOM tree for the document in memory.
 *
 * The stack defines a PHP iterator to traverse it in "reverse order",
 * that is, the most-recently-added element is visited first in a
 * foreach loop.
 *
 * @ingroup Parser
 * @since 1.27
 * @see https://html.spec.whatwg.org/multipage/syntax.html#the-stack-of-open-elements
 */
class BalanceStack implements IteratorAggregate {
	/**
	 * Backing storage for the stack.
	 * @var BalanceElement[] $elements
	 */
	private $elements = [];
	/**
	 * Foster parent mode determines how nodes are inserted into the
	 * stack.
	 * @var bool $fosterParentMode
	 * @see https://html.spec.whatwg.org/multipage/syntax.html#foster-parent
	 */
	public $fosterParentMode = false;
	/**
	 * Configuration options governing flattening.
	 * @var array $config
	 * @see Balancer::__construct()
	 */
	private $config;
	/**
	 * Reference to the current element
	 */
	public $currentNode;

	/**
	 * Create a new BalanceStack with a single BalanceElement on it,
	 * representing the root &lt;html&gt; node.
	 * @param array $config Balancer configuration; see Balancer::_construct().
	 */
	public function __construct( array $config ) {
		// always a root <html> element on the stack
		array_push(
			$this->elements,
			new BalanceElement( BalanceSets::HTML_NAMESPACE, 'html', [] )
		);
		$this->currentNode = $this->elements[0];
		$this->config = $config;
	}

	/**
	 * Return a string representing the output of the tree builder:
	 * all the children of the root &lt;html&gt; node.
	 * @return string
	 */
	public function getOutput() {
		// Don't include the outer '<html>....</html>'
		$out = '';
		foreach ( $this->elements[0]->children as $elt ) {
			$out .= is_string( $elt ) ? $elt :
				$elt->flatten( $this->config );
		}
		return $out;
	}

	/**
	 * Insert a comment at the appropriate place for inserting a node.
	 * @param string $value Content of the comment.
	 * @see https://html.spec.whatwg.org/multipage/syntax.html#insert-a-comment
	 */
	public function insertComment( $value ) {
		// Just another type of text node, except for tidy p-wrapping.
		return $this->insertText( '<!--' . $value . '-->', true );
	}

	/**
	 * Insert text at the appropriate place for inserting a node.
	 * @param string $value
	 * @param bool $isComment
	 * @see https://html.spec.whatwg.org/multipage/syntax.html#appropriate-place-for-inserting-a-node
	 */
	public function insertText( $value, $isComment = false ) {
		if (
			$this->fosterParentMode &&
			$this->currentNode->isA( BalanceSets::$tableSectionRowSet )
		) {
			$this->fosterParent( $value );
		} elseif (
			$this->config['tidyCompat'] && !$isComment &&
			$this->currentNode->isA( BalanceSets::$tidyPWrapSet )
		) {
			$this->insertHTMLElement( 'mw:p-wrap', [] );
			return $this->insertText( $value );
		} else {
			$this->currentNode->appendChild( $value );
		}
	}

	/**
	 * Insert a BalanceElement at the appropriate place, pushing it
	 * on to the open elements stack.
	 * @param string $namespaceURI The element namespace
	 * @param string $tag The tag name
	 * @param string $attribs Normalized attributes, as a string.
	 * @return BalanceElement
	 * @see https://html.spec.whatwg.org/multipage/syntax.html#insert-a-foreign-element
	 */
	public function insertForeignElement( $namespaceURI, $tag, $attribs ) {
		return $this->insertElement(
			new BalanceElement( $namespaceURI, $tag, $attribs )
		);
	}

	/**
	 * Insert an HTML element at the appropriate place, pushing it on to
	 * the open elements stack.
	 * @param string $tag The tag name
	 * @param string $attribs Normalized attributes, as a string.
	 * @return BalanceElement
	 * @see https://html.spec.whatwg.org/multipage/syntax.html#insert-an-html-element
	 */
	public function insertHTMLElement( $tag, $attribs ) {
		return $this->insertForeignElement(
			BalanceSets::HTML_NAMESPACE, $tag, $attribs
		);
	}

	/**
	 * Insert an element at the appropriate place and push it on to the
	 * open elements stack.
	 * @param BalanceElement $elt
	 * @return BalanceElement
	 * @see https://html.spec.whatwg.org/multipage/syntax.html#appropriate-place-for-inserting-a-node
	 */
	public function insertElement( BalanceElement $elt ) {
		if (
			$this->currentNode->isHtmlNamed( 'mw:p-wrap' ) &&
			!$elt->isA( BalanceSets::$tidyInlineSet )
		) {
			// Tidy compatibility.
			$this->pop();
		}
		if (
			$this->fosterParentMode &&
			$this->currentNode->isA( BalanceSets::$tableSectionRowSet )
		) {
			$elt = $this->fosterParent( $elt );
		} else {
			$this->currentNode->appendChild( $elt );
		}
		Assert::invariant( $elt->parent !== null, "$elt must be in tree" );
		Assert::invariant( $elt->parent !== 'flat', "$elt must not have been previous flattened" );
		array_push( $this->elements, $elt );
		$this->currentNode = $elt;
		return $elt;
	}

	/**
	 * Determine if the stack has $tag in scope.
	 * @param BalanceElement|array|string $tag
	 * @return bool
	 * @see https://html.spec.whatwg.org/multipage/syntax.html#has-an-element-in-scope
	 */
	public function inScope( $tag ) {
		return $this->inSpecificScope( $tag, BalanceSets::$inScopeSet );
	}

	/**
	 * Determine if the stack has $tag in button scope.
	 * @param BalanceElement|array|string $tag
	 * @return bool
	 * @see https://html.spec.whatwg.org/multipage/syntax.html#has-an-element-in-button-scope
	 */
	public function inButtonScope( $tag ) {
		return $this->inSpecificScope( $tag, BalanceSets::inButtonScopeSet() );
	}

	/**
	 * Determine if the stack has $tag in list item scope.
	 * @param BalanceElement|array|string $tag
	 * @return bool
	 * @see https://html.spec.whatwg.org/multipage/syntax.html#has-an-element-in-list-item-scope
	 */
	public function inListItemScope( $tag ) {
		return $this->inSpecificScope( $tag, BalanceSets::inListItemScopeSet() );
	}

	/**
	 * Determine if the stack has $tag in table scope.
	 * @param BalanceElement|array|string $tag
	 * @return bool
	 * @see https://html.spec.whatwg.org/multipage/syntax.html#has-an-element-in-table-scope
	 */
	public function inTableScope( $tag ) {
		return $this->inSpecificScope( $tag, BalanceSets::$inTableScopeSet );
	}

	/**
	 * Determine if the stack has $tag in select scope.
	 * @param BalanceElement|array|string $tag
	 * @return bool
	 * @see https://html.spec.whatwg.org/multipage/syntax.html#has-an-element-in-select-scope
	 */
	public function inSelectScope( $tag ) {
		// Can't use inSpecificScope to implement this, since it involves
		// *inverting* a set of tags.  Implement manually.
		foreach ( $this as $elt ) {
			if ( $elt->isA( $tag ) ) {
				return true;
			}
			if ( !$elt->isA( BalanceSets::$inInvertedSelectScopeSet ) ) {
				return false;
			}
		}
		return false;
	}

	/**
	 * Determine if the stack has $tag in a specific scope, $set.
	 * @param BalanceElement|array|string $tag
	 * @param BalanceElement|array|string $set
	 * @return bool
	 * @see https://html.spec.whatwg.org/multipage/syntax.html#has-an-element-in-the-specific-scope
	 */
	public function inSpecificScope( $tag, $set ) {
		foreach ( $this as $elt ) {
			if ( $elt->isA( $tag ) ) {
				return true;
			}
			if ( $elt->isA( $set ) ) {
				return false;
			}
		}
		return false;
	}

	/**
	 * Generate implied end tags.
	 * @param string $butnot
	 * @param bool $thorough True if we should generate end tags thoroughly.
	 * @see https://html.spec.whatwg.org/multipage/syntax.html#generate-implied-end-tags
	 */
	public function generateImpliedEndTags( $butnot = null, $thorough = false ) {
		$endTagSet = $thorough ?
			BalanceSets::$thoroughImpliedEndTagsSet :
			BalanceSets::$impliedEndTagsSet;
		while ( $this->currentNode ) {
			if ( $butnot !== null && $this->currentNode->isHtmlNamed( $butnot ) ) {
				break;
			}
			if ( !$this->currentNode->isA( $endTagSet ) ) {
				break;
			}
			$this->pop();
		}
	}

	/**
	 * Return the adjusted current node.
	 */
	public function adjustedCurrentNode( $fragmentContext ) {
		return ( $fragmentContext && count( $this->elements ) === 1 ) ?
			$fragmentContext : $this->currentNode;
	}

	/**
	 * Return an iterator over this stack which visits the current node
	 * first, and the root node last.
	 * @return \Iterator
	 */
	public function getIterator() {
		return new ReverseArrayIterator( $this->elements );
	}

	/**
	 * Return the BalanceElement at the given position $idx, where
	 * position 0 represents the root element.
	 * @param int $idx
	 * @return BalanceElement
	 */
	public function node( $idx ) {
		return $this->elements[ $idx ];
	}

	/**
	 * Replace the element at position $idx in the BalanceStack with $elt.
	 * @param int $idx
	 * @param BalanceElement $elt
	 */
	public function replaceAt( $idx, BalanceElement $elt ) {
		Assert::precondition(
			$this->elements[$idx]->parent !== 'flat',
			'Replaced element should not have already been flattened.'
		);
		Assert::precondition(
			$elt->parent !== 'flat',
			'New element should not have already been flattened.'
		);
		$this->elements[$idx] = $elt;
		if ( $idx === count( $this->elements ) - 1 ) {
			$this->currentNode = $elt;
		}
	}

	/**
	 * Return the position of the given BalanceElement, set, or
	 * HTML tag name string in the BalanceStack.
	 * @param BalanceElement|array|string $tag
	 * @return int
	 */
	public function indexOf( $tag ) {
		for ( $i = count( $this->elements ) - 1; $i >= 0; $i-- ) {
			if ( $this->elements[$i]->isA( $tag ) ) {
				return $i;
			}
		}
		return -1;
	}

	/**
	 * Return the number of elements currently in the BalanceStack.
	 * @return int
	 */
	public function length() {
		return count( $this->elements );
	}

	/**
	 * Remove the current node from the BalanceStack, flattening it
	 * in the process.
	 */
	public function pop() {
		$elt = array_pop( $this->elements );
		if ( count( $this->elements ) ) {
			$this->currentNode = $this->elements[ count( $this->elements ) - 1 ];
		} else {
			$this->currentNode = null;
		}
		if ( !$elt->isHtmlNamed( 'mw:p-wrap' ) ) {
			$elt->flatten( $this->config );
		}
	}

	/**
	 * Remove all nodes up to and including position $idx from the
	 * BalanceStack, flattening them in the process.
	 * @param int $idx
	 */
	public function popTo( $idx ) {
		for ( $length = count( $this->elements ); $length > $idx; $length-- ) {
			$this->pop();
		}
	}

	/**
	 * Pop elements off the stack up to and including the first
	 * element with the specified HTML tagname (or matching the given
	 * set).
	 * @param BalanceElement|array|string $tag
	 */
	public function popTag( $tag ) {
		while ( $this->currentNode ) {
			if ( $this->currentNode->isA( $tag ) ) {
				$this->pop();
				break;
			}
			$this->pop();
		}
	}

	/**
	 * Pop elements off the stack *not including* the first element
	 * in the specified set.
	 * @param BalanceElement|array|string $set
	 */
	public function clearToContext( $set ) {
		// Note that we don't loop to 0. Never pop the <html> elt off.
		for ( $length = count( $this->elements ); $length > 1; $length-- ) {
			if ( $this->currentNode->isA( $set ) ) {
				break;
			}
			$this->pop();
		}
	}

	/**
	 * Remove the given $elt from the BalanceStack, optionally
	 * flattening it in the process.
	 * @param BalanceElement $elt The element to remove.
	 * @param bool $flatten Whether to flatten the removed element.
	 */
	public function removeElement( BalanceElement $elt, $flatten = true ) {
		Assert::parameter(
			$elt->parent !== 'flat',
			'$elt',
			'$elt should not already have been flattened.'
		);
		Assert::parameter(
			$elt->parent->parent !== 'flat',
			'$elt',
			'The parent of $elt should not already have been flattened.'
		);
		$idx = array_search( $elt, $this->elements, true );
		Assert::parameter( $idx !== false, '$elt', 'must be in stack' );
		array_splice( $this->elements, $idx, 1 );
		if ( $idx === count( $this->elements ) ) {
			$this->currentNode = $this->elements[$idx - 1];
		}
		if ( $flatten ) {
			// serialize $elt into its parent
			// otherwise, it will eventually serialize when the parent
			// is serialized, we just hold onto the memory for its
			// tree of objects a little longer.
			$elt->flatten( $this->config );
		}
		Assert::postcondition(
			array_search( $elt, $this->elements, true ) === false,
			'$elt should no longer be in open elements stack'
		);
	}

	/**
	 * Find $a in the BalanceStack and insert $b after it.
	 * @param BalanceElement $a
	 * @param BalanceElement $b
	 */
	public function insertAfter( BalanceElement $a, BalanceElement $b ) {
		$idx = $this->indexOf( $a );
		Assert::parameter( $idx !== false, '$a', 'must be in stack' );
		if ( $idx === count( $this->elements ) - 1 ) {
			array_push( $this->elements, $b );
			$this->currentNode = $b;
		} else {
			array_splice( $this->elements, $idx + 1, 0, [ $b ] );
		}
	}

	// Fostering and adoption.

	/**
	 * Foster parent the given $elt in the stack of open elements.
	 * @param BalanceElement|string $elt
	 * @return BalanceElement|string
	 *
	 * @see https://html.spec.whatwg.org/multipage/syntax.html#foster-parent
	 */
	private function fosterParent( $elt ) {
		$lastTable = $this->indexOf( 'table' );
		$lastTemplate = $this->indexOf( 'template' );
		$parent = null;

$parent is not used, you could remove the assignment.

		$before = null;

		if ( $lastTemplate >= 0 && ( $lastTable < 0 || $lastTemplate > $lastTable ) ) {
			$parent = $this->elements[$lastTemplate];
		} elseif ( $lastTable >= 0 ) {
			$parent = $this->elements[$lastTable]->parent;
			// Assume all tables have parents, since we're not running scripts!
			Assert::invariant(
				$parent !== null, "All tables should have parents"
			);
			$before = $this->elements[$lastTable];
		} else {
			$parent = $this->elements[0]; // the `html` element.
		}

		if ( $this->config['tidyCompat'] ) {
			if ( is_string( $elt ) ) {
				// We're fostering text: do we need a p-wrapper?
				if ( $parent->isA( BalanceSets::$tidyPWrapSet ) ) {
					$this->insertHTMLElement( 'mw:p-wrap', [] );
					$this->insertText( $elt );
					return $elt;
				}
			} else {
				// We're fostering an element; do we need to merge p-wrappers?
				if ( $elt->isHtmlNamed( 'mw:p-wrap' ) ) {
					$idx = $before ?
						array_search( $before, $parent->children, true ) :
						count( $parent->children );
					$after = $idx > 0 ? $parent->children[$idx - 1] : '';
					if (
						$after instanceof BalanceElement &&
						$after->isHtmlNamed( 'mw:p-wrap' )
					) {
						return $after; // Re-use existing p-wrapper.
					}
				}
			}
		}

		if ( $before ) {
			$parent->insertBefore( $before, $elt );
		} else {
			$parent->appendChild( $elt );
		}
		return $elt;
	}

	/**
	 * Run the "adoption agency algoritm" (AAA) for the given subject
	 * tag name.
	 * @param string $tag The subject tag name.
	 * @param BalanceActiveFormattingElements $afe The current
	 *   active formatting elements list.
	 * @return true if the adoption agency algorithm "did something", false
	 *   if more processing is required by the caller.
	 * @see https://html.spec.whatwg.org/multipage/syntax.html#adoption-agency-algorithm
	 */
	public function adoptionAgency( $tag, $afe ) {
		// If the current node is an HTML element whose tag name is subject,
		// and the current node is not in the list of active formatting
		// elements, then pop the current node off the stack of open
		// elements and abort these steps.
		if (
			$this->currentNode->isHtmlNamed( $tag ) &&
			!$afe->isInList( $this->currentNode )

It seems like $this->currentNode can also be of type null or string; however, MediaWiki\Tidy\BalanceActiveFormattingElements::isInList() does only seem to accept object<MediaWiki\Tidy\BalanceElement>, maybe add an additional type check?

		) {
			$this->pop();
			return true; // no more handling required
		}

		// Outer loop: If outer loop counter is greater than or
		// equal to eight, then abort these steps.
		for ( $outer = 0; $outer < 8; $outer++ ) {
			// Let the formatting element be the last element in the list
			// of active formatting elements that: is between the end of
			// the list and the last scope marker in the list, if any, or
			// the start of the list otherwise, and has the same tag name
			// as the token.
			$fmtElt = $afe->findElementByTag( $tag );

			// If there is no such node, then abort these steps and instead
			// act as described in the "any other end tag" entry below.
			if ( !$fmtElt ) {
				return false; // false means handle by the default case
			}

			// Otherwise, if there is such a node, but that node is not in
			// the stack of open elements, then this is a parse error;
			// remove the element from the list, and abort these steps.
			$index = $this->indexOf( $fmtElt );
			if ( $index < 0 ) {
				$afe->remove( $fmtElt );
				return true;   // true means no more handling required
			}

			// Otherwise, if there is such a node, and that node is also in
			// the stack of open elements, but the element is not in scope,
			// then this is a parse error; ignore the token, and abort
			// these steps.
			if ( !$this->inScope( $fmtElt ) ) {
				return true;
			}

			// Let the furthest block be the topmost node in the stack of
			// open elements that is lower in the stack than the formatting
			// element, and is an element in the special category. There
			// might not be one.
			$furthestBlock = null;
			$furthestBlockIndex = -1;
			$stackLength = $this->length();
			for ( $i = $index+1; $i < $stackLength; $i++ ) {
				if ( $this->node( $i )->isA( BalanceSets::$specialSet ) ) {
					$furthestBlock = $this->node( $i );
					$furthestBlockIndex = $i;
					break;
				}
			}

			// If there is no furthest block, then the UA must skip the
			// subsequent steps and instead just pop all the nodes from the
			// bottom of the stack of open elements, from the current node
			// up to and including the formatting element, and remove the
			// formatting element from the list of active formatting
			// elements.
			if ( !$furthestBlock ) {
				$this->popTag( $fmtElt );
				$afe->remove( $fmtElt );
				return true;
			}

			// Let the common ancestor be the element immediately above
			// the formatting element in the stack of open elements.
			$ancestor = $this->node( $index-1 );

			// Let a bookmark note the position of the formatting
			// element in the list of active formatting elements
			// relative to the elements on either side of it in the
			// list.
			$BOOKMARK = new BalanceElement( '[bookmark]', '[bookmark]', [] );
			$afe->insertAfter( $fmtElt, $BOOKMARK );

			// Let node and last node be the furthest block.
			$node = $furthestBlock;

$node is not used, you could remove the assignment.

			$lastNode = $furthestBlock;
			$nodeIndex = $furthestBlockIndex;
			$isAFE = false;

$isAFE is not used, you could remove the assignment.

			// Inner loop
			for ( $inner = 1; true; $inner++ ) {
				// Let node be the element immediately above node in
				// the stack of open elements, or if node is no longer
				// in the stack of open elements (e.g. because it got
				// removed by this algorithm), the element that was
				// immediately above node in the stack of open elements
				// before node was removed.
				$node = $this->node( --$nodeIndex );

				// If node is the formatting element, then go
				// to the next step in the overall algorithm.
				if ( $node === $fmtElt ) break;

				// If the inner loop counter is greater than three and node
				// is in the list of active formatting elements, then remove
				// node from the list of active formatting elements.
				$isAFE = $afe->isInList( $node );
				if ( $inner > 3 && $isAFE ) {
					$afe->remove( $node );
					$isAFE = false;
				}

				// If node is not in the list of active formatting
				// elements, then remove node from the stack of open
				// elements and then go back to the step labeled inner
				// loop.
				if ( !$isAFE ) {
					// Don't flatten here, since we're about to relocate
					// parts of this $node.
					$this->removeElement( $node, false );
					continue;
				}

				// Create an element for the token for which the
				// element node was created with common ancestor as
				// the intended parent, replace the entry for node
				// in the list of active formatting elements with an
				// entry for the new element, replace the entry for
				// node in the stack of open elements with an entry for
				// the new element, and let node be the new element.
				$newElt = new BalanceElement(
					$node->namespaceURI, $node->localName, $node->attribs );
				$afe->replace( $node, $newElt );
				$this->replaceAt( $nodeIndex, $newElt );
				$node = $newElt;

				// If last node is the furthest block, then move the
				// aforementioned bookmark to be immediately after the
				// new node in the list of active formatting elements.
				if ( $lastNode === $furthestBlock ) {
					$afe->remove( $BOOKMARK );
					$afe->insertAfter( $newElt, $BOOKMARK );
				}

				// Insert last node into node, first removing it from
				// its previous parent node if any.
				$node->appendChild( $lastNode );

				// Let last node be node.
				$lastNode = $node;
			}

			// If the common ancestor node is a table, tbody, tfoot,
			// thead, or tr element, then, foster parent whatever last
			// node ended up being in the previous step, first removing
			// it from its previous parent node if any.
			if (
				$this->fosterParentMode &&
				$ancestor->isA( BalanceSets::$tableSectionRowSet )
			) {
				$this->fosterParent( $lastNode );
			} else {
				// Otherwise, append whatever last node ended up being in
				// the previous step to the common ancestor node, first
				// removing it from its previous parent node if any.
				$ancestor->appendChild( $lastNode );
			}

			// Create an element for the token for which the
			// formatting element was created, with furthest block
			// as the intended parent.
			$newElt2 = new BalanceElement(
				$fmtElt->namespaceURI, $fmtElt->localName, $fmtElt->attribs );

			// Take all of the child nodes of the furthest block and
			// append them to the element created in the last step.
			$newElt2->adoptChildren( $furthestBlock );

			// Append that new element to the furthest block.
			$furthestBlock->appendChild( $newElt2 );

			// Remove the formatting element from the list of active
			// formatting elements, and insert the new element into the
			// list of active formatting elements at the position of
			// the aforementioned bookmark.
			$afe->remove( $fmtElt );
			$afe->replace( $BOOKMARK, $newElt2 );

			// Remove the formatting element from the stack of open
			// elements, and insert the new element into the stack of
			// open elements immediately below the position of the
			// furthest block in that stack.
			$this->removeElement( $fmtElt );
			$this->insertAfter( $furthestBlock, $newElt2 );
		}

		return true;
	}

	/**
	 * Return the contents of the open elements stack as a string for
	 * debugging.
	 * @return string
	 */
	public function __toString() {
		$r = [];
		foreach ( $this->elements as $elt ) {
			array_push( $r, $elt->localName );
		}
		return implode( $r, ' ' );
	}
}

/**
 * A pseudo-element used as a marker in the list of active formatting elements
 *
 * @ingroup Parser
 * @since 1.27
 */
class BalanceMarker {
	public $nextAFE;
	public $prevAFE;
}

/**
 * The list of active formatting elements, which is used to handle
 * mis-nested formatting element tags in the HTML5 tree builder
 * specification.
 *
 * @ingroup Parser
 * @since 1.27
 * @see https://html.spec.whatwg.org/multipage/syntax.html#list-of-active-formatting-elements
 */
class BalanceActiveFormattingElements {
	/** The last (most recent) element in the list */
	private $tail;

	/** The first (least recent) element in the list */
	private $head;

	/**
	 * An array of arrays representing the population of elements in each bucket
	 * according to the Noah's Ark clause. The outer array is stack-like, with each
	 * integer-indexed element representing a segment of the list, bounded by
	 * markers. The first element represents the segment of the list before the
	 * first marker.
	 *
	 * The inner arrays are indexed by "Noah key", which is a string which uniquely
	 * identifies each bucket according to the rules in the spec. The value in
	 * the inner array is the first (least recently inserted) element in the bucket,
	 * and subsequent members of the bucket can be found by iterating through the
	 * singly-linked list via $node->nextNoah.
	 *
	 * This is optimised for the most common case of inserting into a bucket
	 * with zero members, and deleting a bucket containing one member. In the
	 * worst case, iteration through the list is still O(1) in the document
	 * size, since each bucket can have at most 3 members.
	 */
	private $noahTableStack = [ [] ];

	public function __destruct() {
		for ( $node = $this->head; $node; $node = $next ) {
			$next = $node->nextAFE;
			$node->prevAFE = $node->nextAFE = $node->nextNoah = null;
		}
		$this->head = $this->tail = $this->noahTableStack = null;

It seems like null of type null is incompatible with the declared type array of property $noahTableStack.

	}

	public function insertMarker() {
		$elt = new BalanceMarker;
		if ( $this->tail ) {
			$this->tail->nextAFE = $elt;
			$elt->prevAFE = $this->tail;
		} else {
			$this->head = $elt;
		}
		$this->tail = $elt;
		$this->noahTableStack[] = [];
	}

	/**
	 * Follow the steps required when the spec requires us to "push onto the
	 * list of active formatting elements".
	 * @param BalanceElement $elt
	 */
	public function push( BalanceElement $elt ) {
		// Must not be in the list already
		if ( $elt->prevAFE !== null || $this->head === $elt ) {
			throw new ParameterAssertionException( '$elt',
				'Cannot insert a node into the AFE list twice' );
		}

		// "Noah's Ark clause" -- if there are already three copies of
		// this element before we encounter a marker, then drop the last
		// one.
		$noahKey = $elt->getNoahKey();
		$table =& $this->noahTableStack[ count( $this->noahTableStack ) - 1 ];
		if ( !isset( $table[$noahKey] ) ) {
			$table[$noahKey] = $elt;
		} else {
			$count = 1;
			$head = $tail = $table[$noahKey];
			while ( $tail->nextNoah ) {
				$tail = $tail->nextNoah;
				$count++;
			}
			if ( $count >= 3 ) {
				$this->remove( $head );
			}
			$tail->nextNoah = $elt;
		}
		// Add to the main AFE list
		if ( $this->tail ) {
			$this->tail->nextAFE = $elt;
			$elt->prevAFE = $this->tail;
		} else {
			$this->head = $elt;
		}
		$this->tail = $elt;
	}

	/**
	 * Follow the steps required when the spec asks us to "clear the list of
	 * active formatting elements up to the last marker".
	 */
	public function clearToMarker() {
		// Iterate back through the list starting from the tail
		$tail = $this->tail;
		while ( $tail && !( $tail instanceof BalanceMarker ) ) {
			// Unlink the element
			$prev = $tail->prevAFE;
			$tail->prevAFE = null;
			if ( $prev ) {
				$prev->nextAFE = null;
			}
			$tail->nextNoah = null;
			$tail = $prev;
		}
		// If we finished on a marker, unlink it and pop it off the Noah table stack
		if ( $tail ) {
			$prev = $tail->prevAFE;
			if ( $prev ) {
				$prev->nextAFE = null;
			}
			$tail = $prev;
			array_pop( $this->noahTableStack );
		} else {
			// No marker: wipe the top-level Noah table (which is the only one)
			$this->noahTableStack[0] = [];
		}
		// If we removed all the elements, clear the head pointer
		if ( !$tail ) {
			$this->head = null;
		}
		$this->tail = $tail;
	}

	/**
	 * Find and return the last element with the specified tag between the
	 * end of the list and the last marker on the list.
	 * Used when parsing <a> "in body mode".
	 */
	public function findElementByTag( $tag ) {
		$elt = $this->tail;
		while ( $elt && !( $elt instanceof BalanceMarker ) ) {
			if ( $elt->localName === $tag ) {
				return $elt;
			}
			$elt = $elt->prevAFE;
		}
		return null;
	}

	/**
	 * Determine whether an element is in the list of formatting elements.
	 * @param BalanceElement $elt
	 * @return boolean
	 */
	public function isInList( BalanceElement $elt ) {
		return $this->head === $elt || $elt->prevAFE;
	}

	/**
	 * Find the element $elt in the list and remove it.
	 * Used when parsing <a> in body mode.
	 *
	 * @param BalanceElement $elt
	 */
	public function remove( BalanceElement $elt ) {
		if ( $this->head !== $elt && !$elt->prevAFE ) {
			throw new ParameterAssertionException( '$elt',
				"Attempted to remove an element which is not in the AFE list" );
		}
		// Update head and tail pointers
		if ( $this->head === $elt ) {
			$this->head = $elt->nextAFE;
		}
		if ( $this->tail === $elt ) {
			$this->tail = $elt->prevAFE;
		}
		// Update previous element
		if ( $elt->prevAFE ) {
			$elt->prevAFE->nextAFE = $elt->nextAFE;
		}
		// Update next element
		if ( $elt->nextAFE ) {
			$elt->nextAFE->prevAFE = $elt->prevAFE;
		}
		// Clear pointers so that isInList() etc. will work
		$elt->prevAFE = $elt->nextAFE = null;
		// Update Noah list
		$this->removeFromNoahList( $elt );
	}

	private function addToNoahList( BalanceElement $elt ) {
		$noahKey = $elt->getNoahKey();
		$table =& $this->noahTableStack[ count( $this->noahTableStack ) - 1 ];
		if ( !isset( $table[$noahKey] ) ) {
			$table[$noahKey] = $elt;
		} else {
			$tail = $table[$noahKey];
			while ( $tail->nextNoah ) {
				$tail = $tail->nextNoah;
			}
			$tail->nextNoah = $elt;
		}
	}

	private function removeFromNoahList( BalanceElement $elt ) {
		$table =& $this->noahTableStack[ count( $this->noahTableStack ) - 1 ];
		$key = $elt->getNoahKey();
		$noahElt = $table[$key];
		if ( $noahElt === $elt ) {
			if ( $noahElt->nextNoah ) {
				$table[$key] = $noahElt->nextNoah;
				$noahElt->nextNoah = null;
			} else {
				unset( $table[$key] );
			}
		} else {
			do {
				$prevNoahElt = $noahElt;
				$noahElt = $prevNoahElt->nextNoah;
				if ( $noahElt === $elt ) {
					// Found it, unlink
					$prevNoahElt->nextNoah = $elt->nextNoah;
					$elt->nextNoah = null;
					break;
				}
			} while ( $noahElt );
		}
	}

	/**
	 * Find element $a in the list and replace it with element $b
	 *
	 * @param BalanceElement $a
	 * @param BalanceElement $b
	 */
	public function replace( BalanceElement $a, BalanceElement $b ) {
		if ( $this->head !== $a && !$a->prevAFE ) {
			throw new ParameterAssertionException( '$a',
				"Attempted to replace an element which is not in the AFE list" );
		}
		// Update head and tail pointers
		if ( $this->head === $a ) {
			$this->head = $b;
		}
		if ( $this->tail === $a ) {
			$this->tail = $b;
		}
		// Update previous element
		if ( $a->prevAFE ) {
			$a->prevAFE->nextAFE = $b;
		}
		// Update next element
		if ( $a->nextAFE ) {
			$a->nextAFE->prevAFE = $b;
		}
		$b->prevAFE = $a->prevAFE;
		$b->nextAFE = $a->nextAFE;
		$a->nextAFE = $a->prevAFE = null;
		// Update Noah list
		$this->removeFromNoahList( $a );
		$this->addToNoahList( $b );
	}

	/**
	 * Find $a in the list and insert $b after it.

	 * @param BalanceElement $a
	 * @param BalanceElement $b
	 */
	public function insertAfter( BalanceElement $a, BalanceElement $b ) {
		if ( $this->head !== $a && !$a->prevAFE ) {
			throw new ParameterAssertionException( '$a',
				"Attempted to insert after an element which is not in the AFE list" );
		}
		if ( $this->tail === $a ) {
			$this->tail = $b;
		}
		if ( $a->nextAFE ) {
			$a->nextAFE->prevAFE = $b;
		}
		$b->nextAFE = $a->nextAFE;
		$b->prevAFE = $a;
		$a->nextAFE = $b;
		$this->addToNoahList( $b );
	}

	// @codingStandardsIgnoreStart Generic.Files.LineLength.TooLong
	/**
	 * Reconstruct the active formatting elements.
	 * @param BalanceStack $stack The open elements stack
	 * @see https://html.spec.whatwg.org/multipage/syntax.html#reconstruct-the-active-formatting-elements
	 */
	// @codingStandardsIgnoreEnd
	public function reconstruct( $stack ) {
		$entry = $this->tail;
		// If there are no entries in the list of active formatting elements,
		// then there is nothing to reconstruct
		if ( !$entry ) {
			return;
		}
		// If the last is a marker, do nothing.
		if ( $entry instanceof BalanceMarker ) {
			return;
		}
		// Or if it is an open element, do nothing.
		if ( $stack->indexOf( $entry ) >= 0 ) {
			return;
		}

		// Loop backward through the list until we find a marker or an
		// open element
		$foundIt = false;
		while ( $entry->prevAFE ) {
			$entry = $entry->prevAFE;
			if ( $entry instanceof BalanceMarker || $stack->indexOf( $entry ) >= 0 ) {
				$foundIt = true;
				break;
			}
		}

		// Now loop forward, starting from the element after the current one (or
		// the first element if we didn't find a marker or open element),
		// recreating formatting elements and pushing them back onto the list
		// of open elements.
		if ( $foundIt ) {
			$entry = $entry->nextAFE;
		}
		do {
			$newElement = $stack->insertHTMLElement(
				$entry->localName,
				$entry->attribs );
			$this->replace( $entry, $newElement );
			$entry = $newElement->nextAFE;
		} while ( $entry );
	}

	/**
	 * Get a string representation of the AFE list, for debugging
	 */
	public function __toString() {
		$prev = null;
		$s = '';
		for ( $node = $this->head; $node; $prev = $node, $node = $node->nextAFE ) {
			if ( $node instanceof BalanceMarker ) {
				$s .= "MARKER\n";
				continue;
			}
			$s .= $node->localName . '#' . substr( md5( spl_object_hash( $node ) ), 0, 8 );
			if ( $node->nextNoah ) {
				$s .= " (noah sibling: {$node->nextNoah->localName}#" .
					substr( md5( spl_object_hash( $node->nextNoah ) ), 0, 8 ) .
					')';
			}
			if ( $node->nextAFE && $node->nextAFE->prevAFE !== $node ) {
				$s .= " (reverse link is wrong!)";
			}
			$s .= "\n";
		}
		if ( $prev !== $this->tail ) {
			$s .= "(tail pointer is wrong!)\n";
		}
		return $s;
	}
}

/**
 * An implementation of the tree building portion of the HTML5 parsing
 * spec.
 *
 * This is used to balance and tidy output so that the result can
 * always be cleanly serialized/deserialized by an HTML5 parser.  It
 * does *not* guarantee "conforming" output -- the HTML5 spec contains
 * a number of constraints which are not enforced by the HTML5 parsing
 * process.  But the result will be free of gross errors: misnested or
 * unclosed tags, for example, and will be unchanged by spec-complient
 * parsing followed by serialization.
 *
 * The tree building stage is structured as a state machine.
 * When comparing the implementation to
 * https://www.w3.org/TR/html5/syntax.html#tree-construction
 * note that each state is implemented as a function with a
 * name ending in `Mode` (because the HTML spec refers to them
 * as insertion modes).  The current insertion mode is held by
 * the $parseMode property.
 *
 * The following simplifications have been made:
 * - We handle body content only (ie, we start `in body`.)
 * - The document is never in "quirks mode".
 * - All occurrences of < and > have been entity escaped, so we
 *   can parse tags by simply splitting on those two characters.
 *   (This also simplifies the handling of < inside <textarea>.)
 *   The character < must not appear inside comments.
 *   Similarly, all attributes have been "cleaned" and are double-quoted
 *   and escaped.
 * - All null characters are assumed to have been removed.
 * - The following elements are disallowed: <html>, <head>, <body>, <frameset>,
 *   <frame>, <plaintext>, <isindex>, <xmp>, <iframe>,
 *   <noembed>, <noscript>, <script>, <title>.  As a result,
 *   further simplifications can be made:
 *   - `frameset-ok` is not tracked.
 *   - `head element pointer` is not tracked (but presumed non-null)
 *   - Tokenizer has only a single mode. (<textarea> wants RCDATA and
 *     <style>/<noframes> want RAWTEXT modes which we only loosely emulate.)
 *
 *   We generally mark places where we omit cases from the spec due to
 *   disallowed elements with a comment: `// OMITTED: <element-name>`.
 *
 *   The HTML spec keeps a flag during the parsing process to track
 *   whether or not a "parse error" has been encountered.  We don't
 *   bother to track that flag, we just implement the error-handling
 *   process as specified.
 *
 * @ingroup Parser
 * @since 1.27
 * @see https://html.spec.whatwg.org/multipage/syntax.html#tree-construction
 */
class Balancer {
	private $parseMode;
	/** @var \Iterator */
	private $bitsIterator;
	private $allowedHtmlElements;
	/** @var BalanceActiveFormattingElements */
	private $afe;
	/** @var BalanceStack */
	private $stack;
	private $strict;
	private $allowComments;
	private $config;

	private $textIntegrationMode;
	private $pendingTableText;
	private $originalInsertionMode;
	private $fragmentContext;
	private $formElementPointer;
	private $ignoreLinefeed;
	private $inRCDATA;
	private $inRAWTEXT;

	/** @var callable|null */
	private $processingCallback;
	/** @var array */
	private $processingArgs;

	/**
	 * Valid HTML5 comments.
	 * Regex borrowed from Tim Starling's "remex-html" project.
	 */
	const VALID_COMMENT_REGEX = "~ !--
		(                             # 1. Comment match detector
			> | -> | # Invalid short close
			(                         # 2. Comment contents
				(?:
					(?! --> )
					(?! --!> )
					(?! --! \z )
					(?! -- \z )
					(?! - \z )
					.
				)*+
			)
			(                         # 3. Comment close
				--> |   # Normal close
				--!> |  # Comment end bang
				(                     # 4. Indicate matches requiring EOF
					--! |   # EOF in comment end bang state
					-- |    # EOF in comment end state
					-  |    # EOF in comment end dash state
					        # EOF in comment state
				)
			)
		)
		([^<]*) \z                    # 5. Non-tag text after the comment
		~xs";

	/**
	 * Create a new Balancer.
	 * @param array $config Balancer configuration.  Includes:
	 *     'strict' : boolean, defaults to false.
	 *         When true, enforces syntactic constraints on input:
	 *         all non-tag '<' must be escaped, all attributes must be
	 *         separated by a single space and double-quoted.  This is
	 *         consistent with the output of the Sanitizer.
	 *     'allowedHtmlElements' : array, defaults to null.
	 *         When present, the keys of this associative array give
	 *         the acceptable HTML tag names.  When not present, no
	 *         tag sanitization is done.
	 *     'tidyCompat' : boolean, defaults to false.
	 *         When true, the serialization algorithm is tweaked to
	 *         provide historical compatibility with the old "tidy"
	 *         program: <p>-wrapping is done to the children of
	 *         <body> and <blockquote> elements, and empty elements
	 *         are removed.
	 *     'allowComments': boolean, defaults to true.
	 *         When true, allows HTML comments in the input.
	 *         The Sanitizer generally strips all comments, so if you
	 *         are running on sanitized output you can set this to
	 *         false to get a bit more performance.
	 */
	public function __construct( array $config = [] ) {
		$this->config = $config = $config + [
			'strict' => false,
			'allowedHtmlElements' => null,
			'tidyCompat' => false,
			'allowComments' => true,
		];
		$this->allowedHtmlElements = $config['allowedHtmlElements'];
		$this->strict = $config['strict'];
		$this->allowComments = $config['allowComments'];
		if ( $this->allowedHtmlElements !== null ) {
			// Sanity check!
			$bad = array_uintersect_assoc(
				$this->allowedHtmlElements,
				BalanceSets::$unsupportedSet[BalanceSets::HTML_NAMESPACE],
				function( $a, $b ) {
					// Ignore the values (just intersect the keys) by saying
					// all values are equal to each other.
					return 0;
				}
			);
			if ( count( $bad ) > 0 ) {
				$badstr = implode( array_keys( $bad ), ',' );
				throw new ParameterAssertionException(
					'$config',
					'Balance attempted with sanitization including ' .
					"unsupported elements: {$badstr}"
				);
			}
		}
	}

	/**
	 * Return a balanced HTML string for the HTML fragment given by $text,
	 * subject to the caveats listed in the class description.  The result
	 * will typically be idempotent -- that is, rebalancing the output
	 * would result in no change.
	 *
	 * @param string $text The markup to be balanced
	 * @param callable $processingCallback Callback to do any variable or
	 *   parameter replacements in HTML attributes values
	 * @param array|bool $processingArgs Arguments for the processing callback
	 * @return string The balanced markup
	 */
	public function balance( $text, $processingCallback = null, $processingArgs = [] ) {
		$this->parseMode = 'inBodyMode';
		$this->bitsIterator = new ExplodeIterator( '<', $text );
		$this->afe = new BalanceActiveFormattingElements();
		$this->stack = new BalanceStack( $this->config );
		$this->processingCallback = $processingCallback;
		$this->processingArgs = $processingArgs;

It seems like $processingArgs can also be of type boolean. However, the property $processingArgs is declared as type array. Maybe add an additional type check?

		$this->textIntegrationMode =
			$this->ignoreLinefeed =
			$this->inRCDATA =
			$this->inRAWTEXT = false;

		// The stack is constructed with an <html> element already on it.
		// Set this up as a fragment parsed with <body> as the context.
		$this->fragmentContext =
			new BalanceElement( BalanceSets::HTML_NAMESPACE, 'body', [] );
		$this->resetInsertionMode();
		$this->formElementPointer = null;
		for ( $e = $this->fragmentContext; $e != null; $e = $e->parent ) {
			if ( $e->isHtmlNamed( 'form' ) ) {
				$this->formElementPointer = $e;
				break;
			}
		}

		// First element is text not tag
		$x = $this->bitsIterator->current();
		$this->bitsIterator->next();
		$this->insertToken( 'text', str_replace( '>', '&gt;', $x ) );
		// Now process each tag.
		while ( $this->bitsIterator->valid() ) {
			$this->advance();
		}
		$this->insertToken( 'eof', null );
		$result = $this->stack->getOutput();
		// Free memory before returning.
		$this->bitsIterator = null;
		$this->afe = null;
		$this->stack = null;
		$this->fragmentContext = null;
		$this->formElementPointer = null;
		return $result;
	}

	/**
	 * Pass a token to the tree builder.  The $token will be one of the
	 * strings "tag", "endtag", or "text".
	 */
	private function insertToken( $token, $value, $attribs = null, $selfClose = false ) {
		// validate tags against $unsupportedSet
		if ( $token === 'tag' || $token === 'endtag' ) {
			if ( isset( BalanceSets::$unsupportedSet[BalanceSets::HTML_NAMESPACE][$value] ) ) {
				// As described in "simplifications" above, these tags are
				// not supported in the balancer.
				Assert::invariant(
					!$this->strict,
					"Unsupported $token <$value> found."
				);
				return false;
			}
		} elseif ( $token === 'text' && $value === '' ) {
			// Don't actually inject the empty string as a text token.
			return true;
		}
		// Support pre/listing/textarea by suppressing initial linefeed
		if ( $this->ignoreLinefeed ) {
			$this->ignoreLinefeed = false;
			if ( $token === 'text' ) {
				if ( $value[0] === "\n" ) {
					if ( $value === "\n" ) {
						// Nothing would be left, don't inject the empty string.
						return true;
					}
					$value = substr( $value, 1 );
				}
			}
		}
		// Some hoops we have to jump through
		$adjusted = $this->stack->adjustedCurrentNode( $this->fragmentContext );

		$isForeign = true;
		if (
			$this->stack->length() === 0 ||
			$adjusted->isHtml() ||
			$token === 'eof'
		) {
			$isForeign = false;
		} elseif ( $adjusted->isMathmlTextIntegrationPoint() ) {
			if ( $token === 'text' ) {
				$isForeign = false;
			} elseif (
				$token === 'tag' &&
				$value !== 'mglyph' && $value !== 'malignmark'
			) {
				$isForeign = false;
			}
		} elseif (
			$adjusted->namespaceURI === BalanceSets::MATHML_NAMESPACE &&
			$adjusted->localName === 'annotation-xml' &&
			$token === 'tag' && $value === 'svg'
		) {
			$isForeign = false;
		} elseif (
			$adjusted->isHtmlIntegrationPoint() &&
			( $token === 'tag' || $token === 'text' )
		) {
			$isForeign = false;
		}
		if ( $isForeign ) {
			return $this->insertForeignToken( $token, $value, $attribs, $selfClose );
		} else {
			$func = $this->parseMode;
			return $this->$func( $token, $value, $attribs, $selfClose );
		}
	}

	private function insertForeignToken( $token, $value, $attribs = null, $selfClose = false ) {
		if ( $token === 'text' ) {
			$this->stack->insertText( $value );
			return true;
		} elseif ( $token === 'tag' ) {
			switch ( $value ) {
			case 'font':
				if ( isset( $attribs['color'] )
					|| isset( $attribs['face'] )
					|| isset( $attribs['size'] )
				) {
					break;
				}
				// otherwise, fall through
			case 'b':
			case 'big':
			case 'blockquote':
			case 'body':
			case 'br':
			case 'center':
			case 'code':
			case 'dd':
			case 'div':
			case 'dl':
			case 'dt':
			case 'em':
			case 'embed':
			case 'h1':
			case 'h2':
			case 'h3':
			case 'h4':
			case 'h5':
			case 'h6':
			case 'head':
			case 'hr':
			case 'i':
			case 'img':
			case 'li':
			case 'listing':
			case 'menu':
			case 'meta':
			case 'nobr':
			case 'ol':
			case 'p':
			case 'pre':
			case 'ruby':
			case 's':
			case 'small':
			case 'span':
			case 'strong':
			case 'strike':
			case 'sub':
			case 'sup':
			case 'table':
			case 'tt':
			case 'u':
			case 'ul':
			case 'var':
				if ( $this->fragmentContext ) {
					break;
				}
				while ( true ) {
					$this->stack->pop();
					$node = $this->stack->currentNode;
					if (
						$node->isMathmlTextIntegrationPoint() ||
						$node->isHtmlIntegrationPoint() ||
						$node->isHtml()
					) {
						break;
					}
				}
				return $this->insertToken( $token, $value, $attribs, $selfClose );
			}
			// "Any other start tag"
			$adjusted = ( $this->fragmentContext && $this->stack->length()===1 ) ?
				$this->fragmentContext : $this->stack->currentNode;
			$this->stack->insertForeignElement(
				$adjusted->namespaceURI, $value, $attribs
			);
			if ( $selfClose ) {
				$this->stack->pop();
			}
			return true;
		} elseif ( $token === 'endtag' ) {
			$first = true;
			foreach ( $this->stack as $i => $node ) {
				if ( $node->isHtml() && !$first ) {
					// process the end tag as HTML
					$func = $this->parseMode;
					return $this->$func( $token, $value, $attribs, $selfClose );
				} elseif ( $i === 0 ) {
					return true;
				} elseif ( $node->localName === $value ) {
					$this->stack->popTag( $node );
					return true;
				}
				$first = false;
			}
		}
	}

	/**
	 * Grab the next "token" from $bitsIterator.  This is either a open/close
	 * tag or text or a comment, depending on whether the Sanitizer approves.
	 */
	private function advance() {
		$x = $this->bitsIterator->current();
		$this->bitsIterator->next();
		$regs = [];
		// Handle comments.  These won't be generated by mediawiki (they
		// are stripped in the Sanitizer) but may be generated by extensions.
		if (
			$this->allowComments &&
			!( $this->inRCDATA || $this->inRAWTEXT ) &&
			preg_match( Balancer::VALID_COMMENT_REGEX, $x, $regs, PREG_OFFSET_CAPTURE ) &&
			// verify EOF condition where necessary
			( $regs[4][1] < 0 || !$this->bitsIterator->valid() )
		) {
			$contents = $regs[2][0];
			$rest = $regs[5][0];
			$this->insertToken( 'comment', $contents );
			$this->insertToken( 'text', str_replace( '>', '&gt;', $rest ) );
			return;
		}
		// $slash: Does the current element start with a '/'?
		// $t: Current element name
		// $attribStr: String between element name and >
		// $brace: Ending '>' or '/>'
		// $rest: Everything until the next element from the $bitsIterator
		if ( preg_match( Sanitizer::ELEMENT_BITS_REGEX, $x, $regs ) ) {
			list( /* $qbar */, $slash, $t, $attribStr, $brace, $rest ) = $regs;
			$t = strtolower( $t );
			if ( $this->strict ) {
				// Verify that attributes are all properly double-quoted
				Assert::invariant(
					preg_match(
						'/^( [:_A-Z0-9][-.:_A-Z0-9]*="[^"]*")*[ ]*$/i', $attribStr
					),
					"Bad attribute string found"
				);
			}
		} else {
			Assert::invariant(
				!$this->strict, "< found which does not start a valid tag"
			);
			$slash = $t = $attribStr = $brace = $rest = null;
		}
		$goodTag = $t;
		if ( $this->inRCDATA ) {
			if ( $slash && $t === $this->inRCDATA ) {
				$this->inRCDATA = false;
			} else {
				// No tags allowed; this emulates the "rcdata" tokenizer mode.
				$goodTag = false;
			}
		}
		if ( $this->inRAWTEXT ) {
			if ( $slash && $t === $this->inRAWTEXT ) {
				$this->inRAWTEXT = false;
			} else {
				// No tags allowed, no entity-escaping done.
				$goodTag = false;
			}
		}
		$sanitize = $this->allowedHtmlElements !== null;
		if ( $sanitize ) {
			$goodTag = $t && isset( $this->allowedHtmlElements[$t] );

The expression $t of type string|null is loosely compared to true; this is ambiguous if the string can be empty. You might want to explicitly use !== null instead.

		}
		if ( $goodTag ) {
			if ( is_callable( $this->processingCallback ) ) {
				call_user_func_array( $this->processingCallback, [ &$attribStr, $this->processingArgs ] );
			}
			if ( $sanitize ) {
				$goodTag = Sanitizer::validateTag( $attribStr, $t );
			}
		}
		if ( $goodTag ) {
			if ( $sanitize ) {
				$attribs = Sanitizer::decodeTagAttributes( $attribStr );
				$attribs = Sanitizer::validateTagAttributes( $attribs, $t );
			} else {
				$attribs = Sanitizer::decodeTagAttributes( $attribStr );
			}
			$goodTag = $this->insertToken(
				$slash ? 'endtag' : 'tag', $t, $attribs, $brace === '/>'
			);
		}
		if ( $goodTag ) {
			$rest = str_replace( '>', '>', $rest );
			$this->insertToken( 'text', str_replace( '>', '>', $rest ) );
		} elseif ( $this->inRAWTEXT ) {
			$this->insertToken( 'text', "<$x" );
		} else {
			// bad tag; serialize entire thing as text.
			$this->insertToken( 'text', '&lt;' . str_replace( '>', '&gt;', $x ) );
		}
	}

	private function switchMode( $mode ) {
		Assert::parameter(
			substr( $mode, -4 )==='Mode', '$mode', 'should end in Mode'
		);
		$oldMode = $this->parseMode;
		$this->parseMode = $mode;
		return $oldMode;
	}

	private function switchModeAndReprocess( $mode, $token, $value, $attribs, $selfClose ) {
		$this->switchMode( $mode );
		return $this->insertToken( $token, $value, $attribs, $selfClose );
	}

	private function resetInsertionMode() {
		$last = false;
		foreach ( $this->stack as $i => $node ) {
			if ( $i === 0 ) {
				$last = true;
				if ( $this->fragmentContext ) {
					$node = $this->fragmentContext;
				}
			}
			if ( $node->isHtml() ) {
				switch ( $node->localName ) {
				case 'select':
					$stackLength = $this->stack->length();
					for ( $j = $i + 1; $j < $stackLength-1; $j++ ) {
						$ancestor = $this->stack->node( $stackLength-$j-1 );
						if ( $ancestor->isHtmlNamed( 'template' ) ) {
							break;
						}
						if ( $ancestor->isHtmlNamed( 'table' ) ) {
							$this->switchMode( 'inSelectInTableMode' );
							return;
						}
					}
					$this->switchMode( 'inSelectMode' );
					return;
				case 'tr':
					$this->switchMode( 'inRowMode' );
					return;
				case 'tbody':
				case 'tfoot':
				case 'thead':
					$this->switchMode( 'inTableBodyMode' );
					return;
				case 'caption':
					$this->switchMode( 'inCaptionMode' );
					return;
				case 'colgroup':
					$this->switchMode( 'inColumnGroupMode' );
					return;
				case 'table':
					$this->switchMode( 'inTableMode' );
					return;
				case 'template':
					$this->switchMode(
						array_slice( $this->templateInsertionModes, -1 )[0]

The property templateInsertionModes does not exist. Did you maybe forget to declare it?

					);
					return;
				case 'body':
					$this->switchMode( 'inBodyMode' );
					return;
				// OMITTED: <frameset>
				// OMITTED: <html>
				// OMITTED: <head>
				default:
					if ( !$last ) {
						// OMITTED: <head>
						if ( $node->isA( BalanceSets::$tableCellSet ) ) {
							$this->switchMode( 'inCellMode' );
							return;
						}
					}
				}
			}
			if ( $last ) {
				$this->switchMode( 'inBodyMode' );
				return;
			}
		}
	}

	private function stopParsing() {
		// Most of the spec methods are inapplicable, other than step 2:
		// "pop all the nodes off the stack of open elements".
		// We're going to keep the top-most <html> element on the stack, though.

		// Clear the AFE list first, otherwise the element objects will stay live
		// during serialization, potentially using O(N^2) memory. Note that
		// popping the stack will never result in reconstructing the active
		// formatting elements.
		$this->afe = null;
		$this->stack->popTo( 1 );
	}

	private function parseRawText( $value, $attribs = null ) {
		$this->stack->insertHTMLElement( $value, $attribs );
		$this->inRAWTEXT = $value;
		$this->originalInsertionMode = $this->switchMode( 'inTextMode' );
		return true;
	}

	private function inTextMode( $token, $value, $attribs = null, $selfClose = false ) {
		if ( $token === 'text' ) {
			$this->stack->insertText( $value );
			return true;
		} elseif ( $token === 'eof' ) {
			$this->stack->pop();
			return $this->switchModeAndReprocess(
				$this->originalInsertionMode, $token, $value, $attribs, $selfClose
			);
		} elseif ( $token === 'endtag' ) {
			$this->stack->pop();
			$this->switchMode( $this->originalInsertionMode );
			return true;
		}
		return true;
	}

	private function inHeadMode( $token, $value, $attribs = null, $selfClose = false ) {
		if ( $token === 'text' ) {
			if ( preg_match( '/^[\x09\x0A\x0C\x0D\x20]+/', $value, $matches ) ) {
				$this->stack->insertText( $matches[0] );
				$value = substr( $value, strlen( $matches[0] ) );
			}
			if ( strlen( $value ) === 0 ) {
				return true; // All text handled.
			}
			// Fall through to handle non-whitespace below.
		} elseif ( $token === 'tag' ) {
			switch ( $value ) {
			case 'meta':
				// OMITTED: in a full HTML parser, this might change the encoding.
				// falls through
			// OMITTED: <html>
			case 'base':
			case 'basefont':
			case 'bgsound':
			case 'link':
				$this->stack->insertHTMLElement( $value, $attribs );
				$this->stack->pop();
				return true;
			// OMITTED: <title>
			// OMITTED: <noscript>
			case 'noframes':
			case 'style':
				return $this->parseRawText( $value, $attribs );
			// OMITTED: <script>
			case 'template':
				$this->stack->insertHTMLElement( $value, $attribs );
				$this->afe->insertMarker();
				// OMITTED: frameset_ok
				$this->switchMode( 'inTemplateMode' );
				$this->templateInsertionModes[] = $this->parseMode;
				return true;
			// OMITTED: <head>
			}
		} elseif ( $token === 'endtag' ) {
			switch ( $value ) {
			// OMITTED: <head>
			// OMITTED: <body>
			// OMITTED: <html>
			case 'br':
				break; // handle at the bottom of the function
			case 'template':
				if ( $this->stack->indexOf( $value ) < 0 ) {
					return true; // Ignore the token.
				}
				$this->stack->generateImpliedEndTags( null, true /* thorough */ );
				$this->stack->popTag( $value );
				$this->afe->clearToMarker();
				array_pop( $this->templateInsertionModes );
				$this->resetInsertionMode();
				return true;
			default:
				// ignore any other end tag
				return true;
			}
		} elseif ( $token === 'comment' ) {
			$this->stack->insertComment( $value );
			return true;
		}

		// If not handled above
		$this->inHeadMode( 'endtag', 'head' ); // synthetic </head>
		// Then redo this one
		return $this->insertToken( $token, $value, $attribs, $selfClose );
	}

	private function inBodyMode( $token, $value, $attribs = null, $selfClose = false ) {
		if ( $token === 'text' ) {
			$this->afe->reconstruct( $this->stack );
			$this->stack->insertText( $value );
			return true;
		} elseif ( $token === 'eof' ) {
			if ( !empty( $this->templateInsertionModes ) ) {
				return $this->inTemplateMode( $token, $value, $attribs, $selfClose );
			}
			$this->stopParsing();
			return true;
		} elseif ( $token === 'tag' ) {
			switch ( $value ) {
			// OMITTED: <html>
			case 'base':
			case 'basefont':
			case 'bgsound':
			case 'link':
			case 'meta':
			case 'noframes':
			// OMITTED: <script>
			case 'style':
			case 'template':
			// OMITTED: <title>
				return $this->inHeadMode( $token, $value, $attribs, $selfClose );
			// OMITTED: <body>
			// OMITTED: <frameset>

			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 'section':
			case 'summary':
			case 'ul':
				if ( $this->stack->inButtonScope( 'p' ) ) {
					$this->inBodyMode( 'endtag', 'p' );
				}
				$this->stack->insertHTMLElement( $value, $attribs );
				return true;

			case 'h1':
			case 'h2':
			case 'h3':
			case 'h4':
			case 'h5':
			case 'h6':
				if ( $this->stack->inButtonScope( 'p' ) ) {
					$this->inBodyMode( 'endtag', 'p' );
				}
				if ( $this->stack->currentNode->isA( BalanceSets::$headingSet ) ) {
					$this->stack->pop();
				}
				$this->stack->insertHTMLElement( $value, $attribs );
				return true;

			case 'pre':
			case 'listing':
				if ( $this->stack->inButtonScope( 'p' ) ) {
					$this->inBodyMode( 'endtag', 'p' );
				}
				$this->stack->insertHTMLElement( $value, $attribs );
				$this->ignoreLinefeed = true;
				// OMITTED: frameset_ok
				return true;

			case 'form':
				if (
					$this->formElementPointer &&
					$this->stack->indexOf( 'template' ) < 0
				) {
					return true; // in a form, not in a template.
				}
				if ( $this->stack->inButtonScope( "p" ) ) {
					$this->inBodyMode( 'endtag', 'p' );
				}
				$elt = $this->stack->insertHTMLElement( $value, $attribs );
				if ( $this->stack->indexOf( 'template' ) < 0 ) {
					$this->formElementPointer = $elt;
				}
				return true;

			case 'li':
				// OMITTED: frameset_ok
				foreach ( $this->stack as $node ) {
					if ( $node->isHtmlNamed( 'li' ) ) {
						$this->inBodyMode( 'endtag', 'li' );
						break;
					}
					if (
						$node->isA( BalanceSets::$specialSet ) &&
						!$node->isA( BalanceSets::$addressDivPSet )
					) {
						break;
					}
				}
				if ( $this->stack->inButtonScope( 'p' ) ) {
					$this->inBodyMode( 'endtag', 'p' );
				}
				$this->stack->insertHTMLElement( $value, $attribs );
				return true;

			case 'dd':
			case 'dt':
				// OMITTED: frameset_ok
				foreach ( $this->stack as $node ) {
					if ( $node->isHtmlNamed( 'dd' ) ) {
						$this->inBodyMode( 'endtag', 'dd' );
						break;
					}
					if ( $node->isHtmlNamed( 'dt' ) ) {
						$this->inBodyMode( 'endtag', 'dt' );
						break;
					}
					if (
						$node->isA( BalanceSets::$specialSet ) &&
						!$node->isA( BalanceSets::$addressDivPSet )
					) {
						break;
					}
				}
				if ( $this->stack->inButtonScope( 'p' ) ) {
					$this->inBodyMode( 'endtag', 'p' );
				}
				$this->stack->insertHTMLElement( $value, $attribs );
				return true;

			// OMITTED: <plaintext>

			case 'button':
				if ( $this->stack->inScope( 'button' ) ) {
					$this->inBodyMode( 'endtag', 'button' );
					return $this->insertToken( $token, $value, $attribs, $selfClose );
				}
				$this->afe->reconstruct( $this->stack );
				$this->stack->insertHTMLElement( $value, $attribs );
				return true;

			case 'a':
				$activeElement = $this->afe->findElementByTag( 'a' );
				if ( $activeElement ) {
					$this->inBodyMode( 'endtag', 'a' );
					if ( $this->afe->isInList( $activeElement ) ) {
						$this->afe->remove( $activeElement );
						// Don't flatten here, since when we fall
						// through below we might foster parent
						// the new <a> tag inside this one.
						$this->stack->removeElement( $activeElement, false );
					}
				}
				// Falls through
			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->afe->reconstruct( $this->stack );
				$this->afe->push( $this->stack->insertHTMLElement( $value, $attribs ) );
				return true;

			case 'nobr':
				$this->afe->reconstruct( $this->stack );
				if ( $this->stack->inScope( 'nobr' ) ) {
					$this->inBodyMode( 'endtag', 'nobr' );
					$this->afe->reconstruct( $this->stack );
				}
				$this->afe->push( $this->stack->insertHTMLElement( $value, $attribs ) );
				return true;

			case 'applet':
			case 'marquee':
			case 'object':
				$this->afe->reconstruct( $this->stack );
				$this->stack->insertHTMLElement( $value, $attribs );
				$this->afe->insertMarker();
				// OMITTED: frameset_ok
				return true;

			case 'table':
				// The document is never in "quirks mode"; see simplifications
				// above.
				if ( $this->stack->inButtonScope( 'p' ) ) {
					$this->inBodyMode( 'endtag', 'p' );
				}
				$this->stack->insertHTMLElement( $value, $attribs );
				// OMITTED: frameset_ok
				$this->switchMode( 'inTableMode' );
				return true;

			case 'area':
			case 'br':
			case 'embed':
			case 'img':
			case 'keygen':
			case 'wbr':
				$this->afe->reconstruct( $this->stack );
				$this->stack->insertHTMLElement( $value, $attribs );
				$this->stack->pop();
				// OMITTED: frameset_ok
				return true;

			case 'input':
				$this->afe->reconstruct( $this->stack );
				$this->stack->insertHTMLElement( $value, $attribs );
				$this->stack->pop();
				// OMITTED: frameset_ok
				// (hence we don't need to examine the tag's "type" attribute)
				return true;

			case 'menuitem':
			case 'param':
			case 'source':
			case 'track':
				$this->stack->insertHTMLElement( $value, $attribs );
				$this->stack->pop();
				return true;

			case 'hr':
				if ( $this->stack->inButtonScope( 'p' ) ) {
					$this->inBodyMode( 'endtag', 'p' );
				}
				$this->stack->insertHTMLElement( $value, $attribs );
				$this->stack->pop();
				return true;

			case 'image':
				// warts!
				return $this->inBodyMode( $token, 'img', $attribs, $selfClose );

			// OMITTED: <isindex>

			case 'textarea':
				$this->stack->insertHTMLElement( $value, $attribs );
				$this->ignoreLinefeed = true;
				$this->inRCDATA = $value; // emulate rcdata tokenizer mode
				// OMITTED: frameset_ok
				return true;

			// OMITTED: <xmp>
			// OMITTED: <iframe>
			// OMITTED: <noembed>
			// OMITTED: <noscript>

			case 'select':

There must be a comment when fall-through is intentional in a non-empty case body

				$this->afe->reconstruct( $this->stack );
				$this->stack->insertHTMLElement( $value, $attribs );
				switch ( $this->parseMode ) {
				case 'inTableMode':
				case 'inCaptionMode':
				case 'inTableBodyMode':
				case 'inRowMode':
				case 'inCellMode':
					$this->switchMode( 'inSelectInTableMode' );
					return true;
				default:
					$this->switchMode( 'inSelectMode' );
					return true;
				}

			case 'optgroup':
			case 'option':
				if ( $this->stack->currentNode->isHtmlNamed( 'option' ) ) {
					$this->inBodyMode( 'endtag', 'option' );
				}
				$this->afe->reconstruct( $this->stack );
				$this->stack->insertHTMLElement( $value, $attribs );
				return true;

			case 'rb':
			case 'rtc':
				if ( $this->stack->inScope( 'ruby' ) ) {
					$this->stack->generateImpliedEndTags();
				}
				$this->stack->insertHTMLElement( $value, $attribs );
				return true;

			case 'rp':
			case 'rt':
				if ( $this->stack->inScope( 'ruby' ) ) {
					$this->stack->generateImpliedEndTags( 'rtc' );
				}
				$this->stack->insertHTMLElement( $value, $attribs );
				return true;

			case 'math':
				$this->afe->reconstruct( $this->stack );
				// We skip the spec's "adjust MathML attributes" and
				// "adjust foreign attributes" steps, since the browser will
				// do this later when it parses the output and it doesn't affect
				// balancing.
				$this->stack->insertForeignElement(
					BalanceSets::MATHML_NAMESPACE, $value, $attribs
				);
				if ( $selfClose ) {
					// emit explicit </math> tag.
					$this->stack->pop();
				}
				return true;

			case 'svg':
				$this->afe->reconstruct( $this->stack );
				// We skip the spec's "adjust SVG attributes" and
				// "adjust foreign attributes" steps, since the browser will
				// do this later when it parses the output and it doesn't affect
				// balancing.
				$this->stack->insertForeignElement(
					BalanceSets::SVG_NAMESPACE, $value, $attribs
				);
				if ( $selfClose ) {
					// emit explicit </svg> tag.
					$this->stack->pop();
				}
				return true;

			case 'caption':
			case 'col':
			case 'colgroup':
			// OMITTED: <frame>
			case 'head':
			case 'tbody':
			case 'td':
			case 'tfoot':
			case 'th':
			case 'thead':
			case 'tr':
				// Ignore table tags if we're not inTableMode
				return true;
			}

			// Handle any other start tag here
			$this->afe->reconstruct( $this->stack );
			$this->stack->insertHTMLElement( $value, $attribs );
			return true;
		} elseif ( $token === 'endtag' ) {
			switch ( $value ) {
			// </body>,</html> are unsupported.

			case 'template':
				return $this->inHeadMode( $token, $value, $attribs, $selfClose );

			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 'section':
			case 'summary':
			case 'ul':
				// Ignore if there is not a matching open tag
				if ( !$this->stack->inScope( $value ) ) {
					return true;
				}
				$this->stack->generateImpliedEndTags();
				$this->stack->popTag( $value );
				return true;

			case 'form':
				if ( $this->stack->indexOf( 'template' ) < 0 ) {
					$openform = $this->formElementPointer;
					$this->formElementPointer = null;
					if ( !$openform || !$this->stack->inScope( $openform ) ) {
						return true;
					}
					$this->stack->generateImpliedEndTags();
					// Don't flatten yet if we're removing a <form> element
					// out-of-order. (eg. `<form><div></form>`)
					$flatten = ( $this->stack->currentNode === $openform );
					$this->stack->removeElement( $openform, $flatten );
				} else {
					if ( !$this->stack->inScope( 'form' ) ) {
						return true;
					}
					$this->stack->generateImpliedEndTags();
					$this->stack->popTag( 'form' );
				}
				return true;

			case 'p':
				if ( !$this->stack->inButtonScope( 'p' ) ) {
					$this->inBodyMode( 'tag', 'p', [] );
					return $this->insertToken( $token, $value, $attribs, $selfClose );
				}
				$this->stack->generateImpliedEndTags( $value );
				$this->stack->popTag( $value );
				return true;

			case 'li':
				if ( !$this->stack->inListItemScope( $value ) ) {
					return true; // ignore
				}
				$this->stack->generateImpliedEndTags( $value );
				$this->stack->popTag( $value );
				return true;

			case 'dd':
			case 'dt':
				if ( !$this->stack->inScope( $value ) ) {
					return true; // ignore
				}
				$this->stack->generateImpliedEndTags( $value );
				$this->stack->popTag( $value );
				return true;

			case 'h1':
			case 'h2':
			case 'h3':
			case 'h4':
			case 'h5':
			case 'h6':
				if ( !$this->stack->inScope( BalanceSets::$headingSet ) ) {
					return true; // ignore
				}
				$this->stack->generateImpliedEndTags();
				$this->stack->popTag( BalanceSets::$headingSet );
				return true;

			case 'sarcasm':
				// Take a deep breath, then:
				break;

			case 'a':
			case 'b':
			case 'big':
			case 'code':
			case 'em':
			case 'font':
			case 'i':
			case 'nobr':
			case 's':
			case 'small':
			case 'strike':
			case 'strong':
			case 'tt':
			case 'u':
				if ( $this->stack->adoptionAgency( $value, $this->afe ) ) {
					return true; // If we did something, we're done.
				}
				break; // Go to the "any other end tag" case.

			case 'applet':
			case 'marquee':
			case 'object':
				if ( !$this->stack->inScope( $value ) ) {
					return true; // ignore
				}
				$this->stack->generateImpliedEndTags();
				$this->stack->popTag( $value );
				$this->afe->clearToMarker();
				return true;

			case 'br':
				// Turn </br> into <br>
				return $this->inBodyMode( 'tag', $value, [] );
			}

			// Any other end tag goes here
			foreach ( $this->stack as $i => $node ) {
				if ( $node->isHtmlNamed( $value ) ) {
					$this->stack->generateImpliedEndTags( $value );
					$this->stack->popTo( $i ); // including $i
					break;
				} elseif ( $node->isA( BalanceSets::$specialSet ) ) {
					return true; // ignore this close token.
				}
			}
			return true;
		} elseif ( $token === 'comment' ) {
			$this->stack->insertComment( $value );
			return true;
		} else {
			Assert::invariant( false, "Bad token type: $token" );
		}
	}

	private function inTableMode( $token, $value, $attribs = null, $selfClose = false ) {
		if ( $token === 'text' ) {
			if ( $this->textIntegrationMode ) {
				return $this->inBodyMode( $token, $value, $attribs, $selfClose );
			} elseif ( $this->stack->currentNode->isA( BalanceSets::$tableSectionRowSet ) ) {
				$this->pendingTableText = '';
				$this->originalInsertionMode = $this->parseMode;
				return $this->switchModeAndReprocess( 'inTableTextMode',
					$token, $value, $attribs, $selfClose );
			}
			// fall through to default case.
		} elseif ( $token === 'eof' ) {
			$this->stopParsing();
			return true;
		} elseif ( $token === 'tag' ) {
			switch ( $value ) {
			case 'caption':
				$this->afe->insertMarker();
				$this->stack->insertHTMLElement( $value, $attribs );
				$this->switchMode( 'inCaptionMode' );
				return true;
			case 'colgroup':
				$this->stack->clearToContext( BalanceSets::$tableContextSet );
				$this->stack->insertHTMLElement( $value, $attribs );
				$this->switchMode( 'inColumnGroupMode' );
				return true;
			case 'col':
				$this->inTableMode( 'tag', 'colgroup', [] );
				return $this->insertToken( $token, $value, $attribs, $selfClose );
			case 'tbody':
			case 'tfoot':
			case 'thead':
				$this->stack->clearToContext( BalanceSets::$tableContextSet );
				$this->stack->insertHTMLElement( $value, $attribs );
				$this->switchMode( 'inTableBodyMode' );
				return true;
			case 'td':
			case 'th':
			case 'tr':
				$this->inTableMode( 'tag', 'tbody', [] );
				return $this->insertToken( $token, $value, $attribs, $selfClose );
			case 'table':
				if ( !$this->stack->inTableScope( $value ) ) {
					return true; // Ignore this tag.
				}
				$this->inTableMode( 'endtag', $value );
				return $this->insertToken( $token, $value, $attribs, $selfClose );

			case 'style':
			// OMITTED: <script>
			case 'template':
				return $this->inHeadMode( $token, $value, $attribs, $selfClose );

			case 'input':
				if ( !isset( $attribs['type'] ) || strcasecmp( $attribs['type'], 'hidden' ) !== 0 ) {
					break; // Handle this as "everything else"
				}
				$this->stack->insertHTMLElement( $value, $attribs );
				$this->stack->pop();
				return true;

			case 'form':
				if (
					$this->formElementPointer ||
					$this->stack->indexOf( 'template' ) >= 0
				) {
					return true; // ignore this token
				}
				$this->formElementPointer =
					$this->stack->insertHTMLElement( $value, $attribs );
				$this->stack->popTag( $this->formElementPointer );
				return true;
			}
			// Fall through for "anything else" clause.
		} elseif ( $token === 'endtag' ) {
			switch ( $value ) {
			case 'table':
				if ( !$this->stack->inTableScope( $value ) ) {
					return true; // Ignore.
				}
				$this->stack->popTag( $value );
				$this->resetInsertionMode();
				return true;
			// OMITTED: <body>
			case 'caption':
			case 'col':
			case 'colgroup':
			// OMITTED: <html>
			case 'tbody':
			case 'td':
			case 'tfoot':
			case 'th':
			case 'thead':
			case 'tr':
				return true; // Ignore the token.
			case 'template':
				return $this->inHeadMode( $token, $value, $attribs, $selfClose );
			}
			// Fall through for "anything else" clause.
		} elseif ( $token === 'comment' ) {
			$this->stack->insertComment( $value );
			return true;
		}
		// This is the "anything else" case:
		$this->stack->fosterParentMode = true;
		$this->inBodyMode( $token, $value, $attribs, $selfClose );
		$this->stack->fosterParentMode = false;
		return true;
	}

	private function inTableTextMode( $token, $value, $attribs = null, $selfClose = false ) {
		if ( $token === 'text' ) {
			$this->pendingTableText .= $value;
			return true;
		}
		// Non-text token:
		$text = $this->pendingTableText;
		$this->pendingTableText = '';
		if ( preg_match( '/[^\x09\x0A\x0C\x0D\x20]/', $text ) ) {
			// This should match the "anything else" case inTableMode
			$this->stack->fosterParentMode = true;
			$this->inBodyMode( 'text', $text );
			$this->stack->fosterParentMode = false;
		} else {
			// Pending text is just whitespace.
			$this->stack->insertText( $text );
		}
		return $this->switchModeAndReprocess(
			$this->originalInsertionMode, $token, $value, $attribs, $selfClose
		);
	}

	// helper for inCaptionMode
	private function endCaption() {
		if ( !$this->stack->inTableScope( 'caption' ) ) {
			return false;
		}
		$this->stack->generateImpliedEndTags();
		$this->stack->popTag( 'caption' );
		$this->afe->clearToMarker();
		$this->switchMode( 'inTableMode' );
		return true;
	}

	private function inCaptionMode( $token, $value, $attribs = null, $selfClose = false ) {
		if ( $token === 'tag' ) {
			switch ( $value ) {
			case 'caption':
			case 'col':
			case 'colgroup':
			case 'tbody':
			case 'td':
			case 'tfoot':
			case 'th':
			case 'thead':
			case 'tr':
				if ( $this->endCaption() ) {
					$this->insertToken( $token, $value, $attribs, $selfClose );
				}
				return true;
			}
			// Fall through to "anything else" case.
		} elseif ( $token === 'endtag' ) {
			switch ( $value ) {
			case 'caption':
				$this->endCaption();
				return true;
			case 'table':
				if ( $this->endCaption() ) {
					$this->insertToken( $token, $value, $attribs, $selfClose );
				}
				return true;
			case 'body':
			case 'col':
			case 'colgroup':
			// OMITTED: <html>
			case 'tbody':
			case 'td':
			case 'tfoot':
			case 'th':
			case 'thead':
			case 'tr':
				// Ignore the token
				return true;
			}
			// Fall through to "anything else" case.
		}
		// The Anything Else case
		return $this->inBodyMode( $token, $value, $attribs, $selfClose );
	}

	private function inColumnGroupMode( $token, $value, $attribs = null, $selfClose = false ) {
		if ( $token === 'text' ) {
			if ( preg_match( '/^[\x09\x0A\x0C\x0D\x20]+/', $value, $matches ) ) {
				$this->stack->insertText( $matches[0] );
				$value = substr( $value, strlen( $matches[0] ) );
			}
			if ( strlen( $value ) === 0 ) {
				return true; // All text handled.
			}
			// Fall through to handle non-whitespace below.
		} elseif ( $token === 'tag' ) {
			switch ( $value ) {
			// OMITTED: <html>
			case 'col':
				$this->stack->insertHTMLElement( $value, $attribs );
				$this->stack->pop();
				return true;
			case 'template':
				return $this->inHeadMode( $token, $value, $attribs, $selfClose );
			}
			// Fall through for "anything else".
		} elseif ( $token === 'endtag' ) {
			switch ( $value ) {
			case 'colgroup':
				if ( !$this->stack->currentNode->isHtmlNamed( 'colgroup' ) ) {
					return true; // Ignore the token.
				}
				$this->stack->pop();
				$this->switchMode( 'inTableMode' );
				return true;
			case 'col':
				return true; // Ignore the token.
			case 'template':
				return $this->inHeadMode( $token, $value, $attribs, $selfClose );
			}
			// Fall through for "anything else".
		} elseif ( $token === 'eof' ) {
			return $this->inBodyMode( $token, $value, $attribs, $selfClose );
		} elseif ( $token === 'comment' ) {
			$this->stack->insertComment( $value );
			return true;
		}

		// Anything else
		if ( !$this->stack->currentNode->isHtmlNamed( 'colgroup' ) ) {
			return true; // Ignore the token.
		}
		$this->inColumnGroupMode( 'endtag', 'colgroup' );
		return $this->insertToken( $token, $value, $attribs, $selfClose );
	}

	// Helper function for inTableBodyMode
	private function endSection() {
		if ( !(
			$this->stack->inTableScope( 'tbody' ) ||
			$this->stack->inTableScope( 'thead' ) ||
			$this->stack->inTableScope( 'tfoot' )
		) ) {
			return false;
		}
		$this->stack->clearToContext( BalanceSets::$tableBodyContextSet );
		$this->stack->pop();
		$this->switchMode( 'inTableMode' );
		return true;
	}
	private function inTableBodyMode( $token, $value, $attribs = null, $selfClose = false ) {
		if ( $token === 'tag' ) {
			switch ( $value ) {
			case 'tr':
				$this->stack->clearToContext( BalanceSets::$tableBodyContextSet );
				$this->stack->insertHTMLElement( $value, $attribs );
				$this->switchMode( 'inRowMode' );
				return true;
			case 'th':
			case 'td':
				$this->inTableBodyMode( 'tag', 'tr', [] );
				$this->insertToken( $token, $value, $attribs, $selfClose );
				return true;
			case 'caption':
			case 'col':
			case 'colgroup':
			case 'tbody':
			case 'tfoot':
			case 'thead':
				if ( $this->endSection() ) {
					$this->insertToken( $token, $value, $attribs, $selfClose );
				}
				return true;
			}
		} elseif ( $token === 'endtag' ) {
			switch ( $value ) {
			case 'table':
				if ( $this->endSection() ) {
					$this->insertToken( $token, $value, $attribs, $selfClose );
				}
				return true;
			case 'tbody':
			case 'tfoot':
			case 'thead':
				if ( $this->stack->inTableScope( $value ) ) {
					$this->endSection();
				}
				return true;
			// OMITTED: <body>
			case 'caption':
			case 'col':
			case 'colgroup':
			// OMITTED: <html>
			case 'td':
			case 'th':
			case 'tr':
				return true; // Ignore the token.
			}
		}
		// Anything else:
		return $this->inTableMode( $token, $value, $attribs, $selfClose );
	}

	// Helper function for inRowMode
	private function endRow() {
		if ( !$this->stack->inTableScope( 'tr' ) ) {
			return false;
		}
		$this->stack->clearToContext( BalanceSets::$tableRowContextSet );
		$this->stack->pop();
		$this->switchMode( 'inTableBodyMode' );
		return true;
	}
	private function inRowMode( $token, $value, $attribs = null, $selfClose = false ) {
		if ( $token === 'tag' ) {
			switch ( $value ) {
			case 'th':
			case 'td':
				$this->stack->clearToContext( BalanceSets::$tableRowContextSet );
				$this->stack->insertHTMLElement( $value, $attribs );
				$this->switchMode( 'inCellMode' );
				$this->afe->insertMarker();
				return true;
			case 'caption':
			case 'col':
			case 'colgroup':
			case 'tbody':
			case 'tfoot':
			case 'thead':
			case 'tr':
				if ( $this->endRow() ) {
					$this->insertToken( $token, $value, $attribs, $selfClose );
				}
				return true;
			}
		} elseif ( $token === 'endtag' ) {
			switch ( $value ) {
			case 'tr':
				$this->endRow();
				return true;
			case 'table':
				if ( $this->endRow() ) {
					$this->insertToken( $token, $value, $attribs, $selfClose );
				}
				return true;
			case 'tbody':
			case 'tfoot':
			case 'thead':
				if (
					$this->stack->inTableScope( $value ) &&
					$this->endRow()
				) {
					$this->insertToken( $token, $value, $attribs, $selfClose );
				}
				return true;
			// OMITTED: <body>
			case 'caption':
			case 'col':
			case 'colgroup':
			// OMITTED: <html>
			case 'td':
			case 'th':
				return true; // Ignore the token.
			}
		}
		// Anything else:
		return $this->inTableMode( $token, $value, $attribs, $selfClose );
	}

	// Helper for inCellMode
	private function endCell() {
		if ( $this->stack->inTableScope( 'td' ) ) {
			$this->inCellMode( 'endtag', 'td' );
			return true;
		} elseif ( $this->stack->inTableScope( 'th' ) ) {
			$this->inCellMode( 'endtag', 'th' );
			return true;
		} else {
			return false;
		}
	}
	private function inCellMode( $token, $value, $attribs = null, $selfClose = false ) {
		if ( $token === 'tag' ) {
			switch ( $value ) {
			case 'caption':
			case 'col':
			case 'colgroup':
			case 'tbody':
			case 'td':
			case 'tfoot':
			case 'th':
			case 'thead':
			case 'tr':
				if ( $this->endCell() ) {
					$this->insertToken( $token, $value, $attribs, $selfClose );
				}
				return true;
			}
		} elseif ( $token === 'endtag' ) {
			switch ( $value ) {
			case 'td':
			case 'th':
				if ( $this->stack->inTableScope( $value ) ) {
					$this->stack->generateImpliedEndTags();
					$this->stack->popTag( $value );
					$this->afe->clearToMarker();
					$this->switchMode( 'inRowMode' );
				}
				return true;
			// OMITTED: <body>
			case 'caption':
			case 'col':
			case 'colgroup':
			// OMITTED: <html>
				return true;

			case 'table':
			case 'tbody':
			case 'tfoot':
			case 'thead':
			case 'tr':
				if ( $this->stack->inTableScope( $value ) ) {
					$this->stack->generateImpliedEndTags();
					$this->stack->popTag( BalanceSets::$tableCellSet );
					$this->afe->clearToMarker();
					$this->switchMode( 'inRowMode' );
					$this->insertToken( $token, $value, $attribs, $selfClose );
				}
				return true;
			}
		}
		// Anything else:
		return $this->inBodyMode( $token, $value, $attribs, $selfClose );
	}

	private function inSelectMode( $token, $value, $attribs = null, $selfClose = false ) {
		if ( $token === 'text' ) {
			$this->stack->insertText( $value );
			return true;
		} elseif ( $token === 'eof' ) {
			return $this->inBodyMode( $token, $value, $attribs, $selfClose );
		} elseif ( $token === 'tag' ) {
			switch ( $value ) {
			// OMITTED: <html>
			case 'option':
				if ( $this->stack->currentNode->isHtmlNamed( 'option' ) ) {
					$this->stack->pop();
				}
				$this->stack->insertHTMLElement( $value, $attribs );
				return true;
			case 'optgroup':
				if ( $this->stack->currentNode->isHtmlNamed( 'option' ) ) {
					$this->stack->pop();
				}
				if ( $this->stack->currentNode->isHtmlNamed( 'optgroup' ) ) {
					$this->stack->pop();
				}
				$this->stack->insertHTMLElement( $value, $attribs );
				return true;
			case 'select':
				$this->inSelectMode( 'endtag', $value ); // treat it like endtag
				return true;
			case 'input':
			case 'keygen':
			case 'textarea':
				if ( !$this->stack->inSelectScope( 'select' ) ) {
					return true; // ignore token (fragment case)
				}
				$this->inSelectMode( 'endtag', 'select' );
				return $this->insertToken( $token, $value, $attribs, $selfClose );
			case 'script':
			case 'template':
				return $this->inHeadMode( $token, $value, $attribs, $selfClose );
			}
		} elseif ( $token === 'endtag' ) {
			switch ( $value ) {
			case 'optgroup':
				if (
					$this->stack->currentNode->isHtmlNamed( 'option' ) &&
					$this->stack->length() >= 2 &&
					$this->stack->node( $this->stack->length() - 2 )->isHtmlNamed( 'optgroup' )
				) {
					$this->stack->pop();
				}
				if ( $this->stack->currentNode->isHtmlNamed( 'optgroup' ) ) {
					$this->stack->pop();
				}
				return true;
			case 'option':
				if ( $this->stack->currentNode->isHtmlNamed( 'option' ) ) {
					$this->stack->pop();
				}
				return true;
			case 'select':
				if ( !$this->stack->inSelectScope( $value ) ) {
					return true; // fragment case
				}
				$this->stack->popTag( $value );
				$this->resetInsertionMode();
				return true;
			case 'template':
				return $this->inHeadMode( $token, $value, $attribs, $selfClose );
			}
		} elseif ( $token === 'comment' ) {
			$this->stack->insertComment( $value );
			return true;
		}
		// anything else: just ignore the token
		return true;
	}

	private function inSelectInTableMode( $token, $value, $attribs = null, $selfClose = false ) {
		switch ( $value ) {
		case 'caption':
		case 'table':
		case 'tbody':
		case 'tfoot':
		case 'thead':
		case 'tr':
		case 'td':
		case 'th':
			if ( $token === 'tag' ) {
				$this->inSelectInTableMode( 'endtag', 'select' );
				return $this->insertToken( $token, $value, $attribs, $selfClose );
			} elseif ( $token === 'endtag' ) {
				if ( $this->stack->inTableScope( $value ) ) {
					$this->inSelectInTableMode( 'endtag', 'select' );
					return $this->insertToken( $token, $value, $attribs, $selfClose );
				}
				return true;
			}
		}
		// anything else
		return $this->inSelectMode( $token, $value, $attribs, $selfClose );
	}

	private function inTemplateMode( $token, $value, $attribs = null, $selfClose = false ) {
		if ( $token === 'text' || $token === 'comment' ) {
			return $this->inBodyMode( $token, $value, $attribs, $selfClose );
		} elseif ( $token === 'eof' ) {
			if ( $this->stack->indexOf( 'template' ) < 0 ) {
				$this->stopParsing();
			} else {
				$this->stack->popTag( 'template' );
				$this->afe->clearToMarker();
				array_pop( $this->templateInsertionModes );
				$this->resetInsertionMode();
				$this->insertToken( $token, $value, $attribs, $selfClose );
			}
			return true;
		} elseif ( $token === 'tag' ) {
			switch ( $value ) {
			case 'base':
			case 'basefont':
			case 'bgsound':
			case 'link':
			case 'meta':
			case 'noframes':
			// OMITTED: <script>
			case 'style':
			case 'template':
			// OMITTED: <title>
				return $this->inHeadMode( $token, $value, $attribs, $selfClose );

			case 'caption':
			case 'colgroup':
			case 'tbody':
			case 'tfoot':
			case 'thead':
				return $this->switchModeAndReprocess(
					'inTableMode', $token, $value, $attribs, $selfClose
				);

			case 'col':
				return $this->switchModeAndReprocess(
					'inColumnGroupMode', $token, $value, $attribs, $selfClose
				);

			case 'tr':
				return $this->switchModeAndReprocess(
					'inTableBodyMode', $token, $value, $attribs, $selfClose
				);

			case 'td':
			case 'th':
				return $this->switchModeAndReprocess(
					'inRowMode', $token, $value, $attribs, $selfClose
				);
			}
			return $this->switchModeAndReprocess(
				'inBodyMode', $token, $value, $attribs, $selfClose
			);
		} elseif ( $token === 'endtag' ) {
			switch ( $value ) {
			case 'template':
				return $this->inHeadMode( $token, $value, $attribs, $selfClose );
			}
			return true;
		} else {
			Assert::invariant( false, "Bad token type: $token" );
		}
	}
}