SO http://bit.ly/18QlcwB

HTML

<h1> body white @ viewport < 1000px, grey @ viewport 1000px and above </h1>

CSS

body{
    background-color:white;
}

@media (min-device-width: 1200px) {

/* CSS styles for viewports 1000px and up */  
body{
background-color:#ff0;
}  

}


@media (min-device-width: 800px) {

/* CSS styles for viewports 1000px and up */  
body{
background-color:#00f;
}  

}


/* jsfiddle hack to force viewport meta */
</style>
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<style>

JavaScript

var width = (window.innerWidth > 0) ? window.innerWidth : screen.width;
echo width