public function push( $token ) {
    /*
		 * > 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
		 * > attributes as element, then remove the earliest such element from the list of active formatting
		 * > elements. For these purposes, the attributes must be compared as they were when the elements were
		 * > 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.
		 */
    // > Add element to the list of active formatting elements.
    $this->stack[] = $token;
  }