HTML API: Improve implementation of adoption agency algorithm

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

@@ -43,6 +43,22 @@ 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.
+	 *
+	 * Do not use this method; it is meant to be used only by the HTML Processor.
+	 *
+	 * @since 6.7.0
+	 *
+	 * @access private
+	 *
+	 * @param int $index 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( $nth ) {
+		return $this->stack[ $nth - 1 ];
+	}
+
 	/**
 	 * Reports if a specific node is in the stack of active formatting elements.
 	 *

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

@@ -622,12 +622,23 @@ public function remove_node( WP_HTML_Token $token ): bool {
 	 * see WP_HTML_Open_Elements::walk_up().
 	 *
 	 * @since 6.4.0
+	 * @since 6.7.0 Accepts $below_this_node to start traversal below a given node, if it exists.
+	 *
+	 * @param ?WP_HTML_Token $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( $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;
 		}
 	}