public static function css($pro = false)
    {
        if($pro)
        {
            $url = 'pro.fontawesome.com';
            $integrity = 'sha384-Bx4pytHkyTDy3aJKjGkGoHPt3tvv6zlwwjc3iqN7ktaiEMLDPqLSZYts2OjKcBx1';
        }
        else
        {
            $url = 'use.fontawesome.com';
            $integrity = 'sha384-50oBUHEmvpQ+1lW4y57PTFmhCaXp0ML5d60M1M7uH2+nqUivzIebhndOJK28anvf';
        }

        return '<link rel="stylesheet" href="https://' . $url . '/releases/v' .
        self::FA_VERSION . '/css/all.css" integrity="' . $integrity . '" crossorigin="anonymous">';
    }