public static function getSerializerFactories(): FluentDOM\Serializer\Factory\Group {
    if (NULL === self::$_serializerFactories) {
      $xml = function($contentType, \DOMNode $node) {
        return new FluentDOM\Serializer\Xml($node);
      };
      $html = function($contentType, \DOMNode $node) {
        return new FluentDOM\Serializer\Html($node);
      };
      $json = function($contentType, \DOMNode $node) {
        return new FluentDOM\Serializer\Json($node);
      };
      self::$_serializerFactories = new FluentDOM\Serializer\Factory\Group(
        [
          'text/html' => $html,
          'html' => $html,
          'text/xml' => $xml,
          'xml' => $xml,
          'text/json' => $json,
          'json' => $json
        ]
      );
    }
    return self::$_serializerFactories;
  }