HTML API: Introduce full parsing mode in HTML Processor.

The HTML Processor has only supported a specific kind of parsing mode
called _the fragment parsing mode_, where it behaves in the same way
that `node.innerHTML = html` does in the DOM. This mode assumes a
context node and doesn't support parsing an entire document.

As part of work to add more spec support to the HTML API, this patch
introduces a full parsing mode, which can parse a full HTML document
from start to end, including the doctype declaration and head tags.

Developed in WordPress#6977
Discussed in https://core.trac.wordpress.org/ticket/61576

Props: dmsnell, jonsurrell.
See #61576.


git-svn-id: https://develop.svn.wordpress.org/trunk@58836 602fd350-edb4-49c9-b593-d223f7449a82

Author: dmsnell

src/wp-includes/html-api/class-wp-html-processor-state.php

@@ -428,6 +428,38 @@ class WP_HTML_Processor_State {
 	 */
 	public $context_node = null;
 
+	/**
+	 * The recognized encoding of the input byte stream.
+	 *
+	 * > The stream of code points that comprises the input to the tokenization
+	 * > stage will be initially seen by the user agent as a stream of bytes
+	 * > (typically coming over the network or from the local file system).
+	 * > The bytes encode the actual characters according to a particular character
+	 * > encoding, which the user agent uses to decode the bytes into characters.
+	 *
+	 * @since 6.7.0
+	 *
+	 * @var string|null
+	 */
+	public $encoding = null;
+
+	/**
+	 * The parser's confidence in the input encoding.
+	 *
+	 * > When the HTML parser is decoding an input byte stream, it uses a character
+	 * > encoding and a confidence. The confidence is either tentative, certain, or
+	 * > irrelevant. The encoding used, and whether the confidence in that encoding
+	 * > is tentative or certain, is used during the parsing to determine whether to
+	 * > change the encoding. If no encoding is necessary, e.g. because the parser is
+	 * > operating on a Unicode stream and doesn't have to use a character encoding
+	 * > at all, then the confidence is irrelevant.
+	 *
+	 * @since 6.7.0
+	 *
+	 * @var string
+	 */
+	public $encoding_confidence = 'tentative';
+
 	/**
 	 * HEAD element pointer.
 	 *