public function class_list() {
    
/** @var string $class contains the string value of the class attribute, with character references decoded. */
    $class = $this->get_attribute( 'class' );

    if ( ! is_string( $class ) ) {
      return;
    }

    $seen = array();

    $at = 0;
    while ( $at < strlen( $class ) ) {
      // Skip past any initial boundary characters.
      $at += strspn( $class, " \t\f\r\n", $at );
      if ( $at >= strlen( $class ) ) {
        return;
      }

      // Find the byte length until the next boundary.
      $length = strcspn( $class, " \t\f\r\n", $at );
      if ( 0 === $length ) {
        return;
      }

      /*
			 * CSS class names are case-insensitive in the ASCII range.
			 *
			 * @see https://www.w3.org/TR/CSS2/syndata.html#x1
			 */
      $name = strtolower( substr( $class, $at, $length ) );
      $at  += $length;

      /*
			 * It's expected that the number of class names for a given tag is relatively small.
			 * Given this, it is probably faster overall to scan an array for a value rather
			 * than to use the class name as a key and check if it's a key of $seen.
			 */
      if ( in_array( $name, $seen, true ) ) {
        continue;
      }

      $seen[] = $name;
      yield $name;
    }
  }