HTML API: Support SELECT insertion mode.

As part of work to add more spec support to the HTML API, this patch adds
support for the SELECT, OPTION, and OPTGROUP elements, including the
requisite support for the IN SELECT insertion mode.

Developed in WordPress#5908
Discussed in https://core.trac.wordpress.org/ticket/61576

Props dmsnell, jonsurrell.
See #61576.


git-svn-id: https://develop.svn.wordpress.org/trunk@58677 602fd350-edb4-49c9-b593-d223f7449a82

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

@@ -189,11 +189,6 @@ public function current_node_is( string $identity ): bool {
 	/**
 	 * Returns whether an element is in a specific scope.
 	 *
-	 * ## HTML Support
-	 *
-	 * This function skips checking for the termination list because there
-	 * are no supported elements which appear in the termination list.
-	 *
 	 * @since 6.4.0
 	 *
 	 * @see https://html.spec.whatwg.org/#has-an-element-in-the-specific-scope
@@ -311,19 +306,38 @@ public function has_element_in_table_scope( $tag_name ) {
 	/**
 	 * Returns whether a particular element is in select scope.
 	 *
-	 * @since 6.4.0
+	 * This test differs from the others like it, in that its rules are inverted.
+	 * Instead of arriving at a match when one of any tag in a termination group
+	 * is reached, this one terminates if any other tag is reached.
 	 *
-	 * @see https://html.spec.whatwg.org/#has-an-element-in-select-scope
+	 * > The stack of open elements is said to have a particular element in select scope when it has
+	 * > that element in the specific scope consisting of all element types except the following:
+	 * >   - optgroup in the HTML namespace
+	 * >   - option in the HTML namespace
 	 *
-	 * @throws WP_HTML_Unsupported_Exception Always until this function is implemented.
+	 * @since 6.4.0 Stub implementation (throws).
+	 * @since 6.7.0 Full implementation.
+	 *
+	 * @see https://html.spec.whatwg.org/#has-an-element-in-select-scope
 	 *
 	 * @param string $tag_name Name of tag to check.
-	 * @return bool Whether given element is in scope.
+	 * @return bool Whether the given element is in SELECT scope.
 	 */
 	public function has_element_in_select_scope( $tag_name ) {
-		throw new WP_HTML_Unsupported_Exception( 'Cannot process elements depending on select scope.' );
+		foreach ( $this->walk_up() as $node ) {
+			if ( $node->node_name === $tag_name ) {
+				return true;
+			}
 
-		return false; // The linter requires this unreachable code until the function is implemented and can return.
+			if (
+				'OPTION' !== $node->node_name &&
+				'OPTGROUP' !== $node->node_name
+			) {
+				return false;
+			}
+		}
+
+		return false;
 	}
 
 	/**