TML document. In most places, they are
	 * simply ignored. The main parsing functions find the basic shape of DOCTYPE tokens but
	 * do not perform detailed parsing.
	 *
	 * This method can be called to perform a full parse of the DOCTYPE token and retrieve
	 * its information.
	 *
	 * @return WP_HTML_Doctype_Info|null The DOCTYPE declaration information or `null` if not
	 *                                   currently at a DOCTYPE node.
	 */
	public function get_doctype_info(): ?WP_HTML_Doctype_Info {
		if ( self::STATE_DOCTYPE !== $this->parser_state ) {
			return null;
		}

		return WP_HTML_Doctype_Info::from_doctype_token( substr( $this->html, $this->token_starts_at, $this->token_length ) );
	}

	/**
	 * Parser Ready State.
	 *
	 * Indicates that the parser is ready to run and waiting for a state transition.
	 * It may not have started yet, or it may have just finished parsing a token and
	 * is ready to find the next one.
	 *
	 * @since 6.5.0
	 *
	 * @access private
	 */
	const STATE_READY = 'STATE_READY';

	/**
	 * Parser Complete State.
	 *
	 * Indicates that the parser has reached the end of the document and there is
	 * nothing left to scan. It finished parsing the last token completely.
	 *
	 * @since 6.5.0
	 *
	 * @access private
	 */
	const STATE_COMPLETE = 'STATE_COMPLETE';

	/**
	 * Parser Incomplete Input State.
	 *
	 * Indicates that the parser has reached the end of the document before finishing
	 * a token. It started parsing a token but there is a possibility that the input
	 * HTML document was truncated in the middle of a token.
	 *
	 * The parser is reset at the start of the incomplete token and has paused. There
	 * is nothing more than can be scanned unless provided a more complete document.
	 *
	 * @since 6.5.0
	 *
	 * @access private
	 */
	const STATE_INCOMPLETE_INPUT = 'STATE_INCOMPLETE_INPUT';

	/**
	 * Parser Matched Tag State.
	 *
	 * Indicates that the parser has found an HTML tag and it's possible to get
	 * the tag name and read or modify its attributes (if it's not a closing tag).
	 *
	 * @since 6.5.0
	 *
	 * @access private
	 */
	const STATE_MATCHED_TAG = 'STATE_MATCHED_TAG';

	/**
	 * Parser Text Node State.
	 *
	 * Indicates that the parser has found a text node and it's possible
	 * to read and modify that text.
	 *
	 * @since 6.5.0
	 *
	 * @access private
	 */
	const STATE_TEXT_NODE = 'STATE_TEXT_NODE';

	/**
	 * Parser CDATA Node State.
	 *
	 * Indicates that the parser has found a CDATA node and it's possible
	 * to read and modify its modifiable text. Note that in HTML there are
	 * no CDATA nodes outside of foreign content (SVG and MathML). Outside
	 * of foreign content, they are treated as HTML comments.
	 *
	 * @since 6.5.0
	 *
	 * @access private
	 */
	const STATE_CDATA_NODE = 'STATE_CDATA_NODE';

	/**
	 * Indicates that the parser has found an HTML comment and it's
	 * possible to read and modify its modifiable text.
	 *
	 * @since 6.5.0
	 *
	 * @access private
	 */
	const STATE_COMMENT = 'STATE_COMMENT';

	/**
	 * Indicates that the parser has found a DOCTYPE node and it's
	 * possible to read its DOCTYPE information via `get_doctype_info()`.
	 *
	 * @since 6.5.0
	 *
	 * @access private
	 */
	const STATE_DOCTYPE = 'STATE_DOCTYPE';

	/**
	 * Indicates that the parser has found an empty tag closer `</>`.
	 *
	 * Note that in HTML there are no empty tag closers, and they
	 * are ignored. Nonetheless, the Tag Processor still
	 * recognizes them as they appear in the HTML stream.
	 *
	 * These were historically discussed as a "presumptuous tag
	 * closer," which would close the nearest open tag, but were
	 * dismissed in favor of explicitly-closing tags.
	 *
	 * @since 6.5.0
	 *
	 * @access private
	 */
	const STATE_PRESUMPTUOUS_TAG = 'STATE_PRESUMPTUOUS_TAG';

	/**
	 * Indicates that the parser has found a "funky comment"
	 * and it's possible to read and modify its modifiable text.
	 *
	 * Example:
	 *
	 *     </%url>
	 *     </{"wp-bit":"query/post-author"}>
	 *     </2>
	 *
	 * Funky comments are tag closers with invalid tag names. Note
	 * that in HTML these are turn into bogus comments. Nonetheless,
	 * the Tag Processor recognizes them in a stream of HTML and
	 * exposes them for inspection and modification.
	 *
	 * @since 6.5.0
	 *
	 * @access private
	 */
	const STATE_FUNKY_COMMENT = 'STATE_WP_FUNKY';

	/**
	 * Indicates that a comment was created when encountering abruptly-closed HTML comment.
	 *
	 * Example:
	 *
	 *     <!-->
	 *     <!--->
	 *
	 * @since 6.5.0
	 */
	const COMMENT_AS_ABRUPTLY_CLOSED_COMMENT = 'COMMENT_AS_ABRUPTLY_CLOSED_COMMENT';

	/**
	 * Indicates that a comment would be parsed as a CDATA node,
	 * were HTML to allow CDATA nodes outside of foreign content.
	 *
	 * Example:
	 *
	 *     <![CDATA[This is a CDATA node.]]>
	 *
	 * This is an HTML comment, but it looks like a CDATA node.
	 *
	 * @since 6.5.0
	 */
	const COMMENT_AS_CDATA_LOOKALIKE = 'COMMENT_AS_CDATA_LOOKALIKE';

	/**
	 * Indicates that a comment was created when encountering
	 * normative HTML comment syntax.
	 *
	 * Example:
	 *
	 *     <!-- this is a comment -->
	 *
	 * @since 6.5.0
	 */
	const COMMENT_AS_HTML_COMMENT = 'COMMENT_AS_HTML_COMMENT';

	/**
	 * Indicates that a comment would be parsed as a Processing
	 * Instruction node, were they to exist within HTML.
	 *
	 * Example:
	 *
	 *     <?wp __( 'Like' ) ?>
	 *
	 * This is an HTML comment, but it looks like a CDATA node.
	 *
	 * @since 6.5.0
	 */
	const COMMENT_AS_PI_NODE_LOOKALIKE = 'COMMENT_AS_PI_NODE_LOOKALIKE';

	/**
	 * Indicates that a comment was created when encountering invalid
	 * HTML input, a so-called "bogus comment."
	 *
	 * Example:
	 *
	 *     <?nothing special>
	 *     <!{nothing special}>
	 *
	 * @since 6.5.0
	 */
	const COMMENT_AS_INVALID_HTML = 'COMMENT_AS_INVALID_HTML';

	/**
	 * No-quirks mode document compatability mode.
	 *
	 * > In no-quirks mode, the behavior is (hopefully) the desired behavior
	 * > described by the modern HTML and CSS specifications.
	 *
	 * @see self::$compat_mode
	 * @see https://developer.mozilla.org/en-US/docs/Web/HTML/Quirks_Mode_and_Standards_Mode
	 *
	 * @since 6.7.0
	 *
	 * @var string
	 */
	const NO_QUIRKS_MODE = 'no-quirks-mode';

	/**
	 * Quirks mode document compatability mode.
	 *
	 * > In quirks mode, layout emulates behavior in Navigator 4 and Internet
	 * > Explorer 5. This is essential in order to support websites that were
	 * > built before the widespread adoption of web standards.
	 *
	 * @see self::$compat_mode
	 * @see https://developer.mozilla.org/en-US/docs/Web/HTML/Quirks_Mode_and_Standards_Mode
	 *
	 * @since 6.7.0
	 *
	 * @var string
	 */
	const QUIRKS_MODE = 'quirks-mode';

	/**
	 * Indicates that a span of text may contain any combination of significant
	 * kinds of characters: NULL bytes, whitespace, and others.
	 *
	 * @see self::$text_node_classification
	 * @see self::subdivide_text_appropriately
	 *
	 * @since 6.7.0
	 */
	const TEXT_IS_GENERIC = 'TEXT_IS_GENERIC';

	/**
	 * Indicates that a span of text comprises a sequence only of NULL bytes.
	 *
	 * @see self::$text_node_classification
	 * @see self::subdivide_text_appropriately
	 *
	 * @since 6.7.0
	 */
	const TEXT_IS_NULL_SEQUENCE = 'TEXT_IS_NULL_SEQUENCE';

	/**
	 * Indicates that a span of decoded text comprises only whitespace.
	 *
	 * @see self::$text_node_classification
	 * @see self::subdivide_text_appropriately
	 *
	 * @since 6.7.0
	 */
	const TEXT_IS_WHITESPACE = 'TEXT_IS_WHITESPACE';
}