HTML API: Reconstruct active formatting elements

src/wp-includes/html-api/class-wp-html-active-formatting-elements.php

@@ -44,6 +44,18 @@ class WP_HTML_Active_Formatting_Elements {
 	 */
 	private $stack = array();
 
+	/**
+	 * Returns the node at the given 1-offset index in the list of active formatting elements.
+	 *
+	 * @since 7.0.0
+	 *
+	 * @param int $nth Number of nodes from the top node to return.
+	 * @return WP_HTML_Token|null Node at the given index in the stack, if one exists, otherwise null.
+	 */
+	public function at( int $nth ): ?WP_HTML_Token {
+		return $this->stack[ $nth - 1 ] ?? null;
+	}
+
 	/**
 	 * Reports if a specific node is in the stack of active formatting elements.
 	 *
@@ -111,8 +123,9 @@ public function insert_marker(): void {
 	 * @see https://html.spec.whatwg.org/#push-onto-the-list-of-active-formatting-elements
 	 *
 	 * @param WP_HTML_Token $token Push this node onto the stack.
+	 * @return bool Whether a node was pushed onto the stack of active formatting elements.
 	 */
-	public function push( WP_HTML_Token $token ) {
+	public function push( WP_HTML_Token $token ): bool {
 		/*
 		 * > If there are already three elements in the list of active formatting elements after the last marker,
 		 * > if any, or anywhere in the list if there are no markers, that have the same tag name, namespace, and
@@ -121,11 +134,31 @@ public function push( WP_HTML_Token $token ) {
 		 * > created by the parser; two elements have the same attributes if all their parsed attributes can be
 		 * > paired such that the two attributes in each pair have identical names, namespaces, and values
 		 * > (the order of the attributes does not matter).
-		 *
-		 * @todo Implement the "Noah's Ark clause" to only add up to three of any given kind of formatting elements to the stack.
 		 */
+		if ( 'marker' !== $token->node_name ) {
+			$existing_count = 0;
+			foreach ( $this->walk_up() as $item ) {
+				if ( 'marker' === $item->node_name ) {
+					break;
+				}
+
+				if (
+					$item->node_name === $token->node_name &&
+					$item->namespace === $token->namespace
+					// @todo Compare attributes. For now, bail if there are three matching tag names + namespaces.
+				) {
+					++$existing_count;
+					if ( $existing_count >= 3 ) {
+						// @todo Implement removing the earliest element and moving forward.
+						return false;
+					}
+				}
+			}
+		}
+
 		// > Add element to the list of active formatting elements.
 		$this->stack[] = $token;
+		return true;
 	}
 
 	/**

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

@@ -614,12 +614,23 @@ public function remove_node( WP_HTML_Token $token ): bool {
 	 * see WP_HTML_Open_Elements::walk_up().
 	 *
 	 * @since 6.4.0
+	 * @since 7.0.0 Accepts $below_this_node to start traversal below a given node, if it exists.
+	 *
+	 * @param WP_HTML_Token|null $below_this_node Start traversing below this node, if provided and if the node exists.
 	 */
-	public function walk_down() {
-		$count = count( $this->stack );
+	public function walk_down( ?WP_HTML_Token $below_this_node = null ) {
+		$has_found_node = null === $below_this_node;
+		$count          = count( $this->stack );
 
 		for ( $i = 0; $i < $count; $i++ ) {
-			yield $this->stack[ $i ];
+			$node = $this->stack[ $i ];
+
+			if ( ! $has_found_node ) {
+				$has_found_node = $node === $below_this_node;
+				continue;
+			}
+
+			yield $node;
 		}
 	}