Сергей Горячев

<div class ="header">
    <div class = "wrapper wrapper--header">
        <div class = "header__content">
            my header
        </div>
    </div>
</div>
<div class ="main">
    <div class = "wrapper wrapper--main">
        <div class = "main__content">
            my main
        </div>
    </div>
</div>

<meta name="viewport" content="width=device-width, initial-scale=1.0, maximum-scale=1.0, user-scalable=no">

<meta name="viewport" content="width=1300px, initial-scale=1.0, maximum-scale=1.0, user-scalable=no">

if(isset($var) && $var !== '')  {
// $var не null и не  ''.
}