<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
</head>
<body>
<div id="col">545</div>
<script>
var media = '@media only screen and (max-width:450px) {#col{background-color: red;}}';
var s = document.createElement('style');
s.innerHTML = media;
document.getElementsByTagName('head')[0].appendChild(s);
</script>
</body>
</html>
Так например