protected function prefixAlignSelf( $value, $extra = null )
  {
    $prop = array(
        '-webkit-align-self',
        '-ms-flex-item-align',
        'align-self'
    );

    $val[] = $value;
    switch( $value )
    {
      case 'flex-start':
        $val[] = 'start';
        break;
      case 'flex-end':
        $val[] = 'end';
        break;
      default:
        $val[] = $value;
        break;
    }

    $val[] = $value;

    return $this->getPrefixRules( $prop, $val, $extra );
  }