protected function prefixFlex( $value, $extra = null )
  {
    $parts = explode( " ", $value );
    if( count( $parts ) == 3 )
    {
      // Make sure there's a % after the basis to avoid IE10/11 Bugs.
      // 0px does not work because a minifier would remove it.
      //
      // https://github.com/philipwalton/flexbugs#4-flex-shorthand-declarations-with-unitless-flex-basis-values-are-ignored
      if( $parts[ 2 ] === 0 || $parts[ 2 ] === '0px' )
      {
        $parts[ 2 ] = '0%';
      }
    }

    $prop = array( '-webkit-flex', '-ms-flex', 'flex' );
    $val  = implode( ' ', $parts );

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