HTML API: Add TEMPLATE and related support in HTML Processor.

As part of work to add more spec support to the HTML API, this patch adds
support for the IN TEMPLATE and IN HEAD insertion modes. These changes are
primarily about adding support for TEMPLATE elements in the HTML Processor,
but include support for other tags commonly found in the document head, such
as LINK, META, SCRIPT, STYLE, and TITLE.

Developed in WordPress#7046
Discussed in https://core.trac.wordpress.org/ticket/61576

Props: dmsnell, jonsurrell, westonruter.
See #61576.


git-svn-id: https://develop.svn.wordpress.org/trunk@58833 602fd350-edb4-49c9-b593-d223f7449a82

Author: dmsnell

src/wp-includes/html-api/class-wp-html-open-elements.php

@@ -308,7 +308,20 @@ public function has_element_in_scope( string $tag_name ): bool {
 				'MARQUEE',
 				'OBJECT',
 				'TEMPLATE',
-				// @todo: Support SVG and MathML nodes when support for foreign content is added.
+
+				/*
+				 * @todo Support SVG and MathML nodes when support for foreign content is added.
+				 *
+				 * - MathML mi
+				 * - MathML mo
+				 * - MathML mn
+				 * - MathML ms
+				 * - MathML mtext
+				 * - MathML annotation-xml
+				 * - SVG foreignObject
+				 * - SVG desc
+				 * - SVG title
+				 */
 			)
 		);
 	}
@@ -349,7 +362,20 @@ public function has_element_in_list_item_scope( string $tag_name ): bool {
 				'OL',
 				'TEMPLATE',
 				'UL',
-				// @todo: Support SVG and MathML nodes when support for foreign content is added.
+
+				/*
+				 * @todo Support SVG and MathML nodes when support for foreign content is added.
+				 *
+				 * - MathML mi
+				 * - MathML mo
+				 * - MathML mn
+				 * - MathML ms
+				 * - MathML mtext
+				 * - MathML annotation-xml
+				 * - SVG foreignObject
+				 * - SVG desc
+				 * - SVG title
+				 */
 			)
 		);
 	}
@@ -386,7 +412,20 @@ public function has_element_in_button_scope( string $tag_name ): bool {
 				'MARQUEE',
 				'OBJECT',
 				'TEMPLATE',
-				// @todo: Support SVG and MathML nodes when support for foreign content is added.
+
+				/*
+				 * @todo Support SVG and MathML nodes when support for foreign content is added.
+				 *
+				 * - MathML mi
+				 * - MathML mo
+				 * - MathML mn
+				 * - MathML ms
+				 * - MathML mtext
+				 * - MathML annotation-xml
+				 * - SVG foreignObject
+				 * - SVG desc
+				 * - SVG title
+				 */
 			)
 		);
 	}