Find the exact height and width of the viewport in a cross-browser way (no Prototype/jQuery)

You might try this:

function getViewport() {

 var viewPortWidth;
 var viewPortHeight;

 // the more standards compliant browsers (mozilla/netscape/opera/IE7) use window.innerWidth and window.innerHeight
 if (typeof window.innerWidth != 'undefined') {
   viewPortWidth = window.innerWidth,
   viewPortHeight = window.innerHeight
 }

// IE6 in standards compliant mode (i.e. with a valid doctype as the first line in the document)
 else if (typeof document.documentElement != 'undefined'
 && typeof document.documentElement.clientWidth !=
 'undefined' && document.documentElement.clientWidth != 0) {
    viewPortWidth = document.documentElement.clientWidth,
    viewPortHeight = document.documentElement.clientHeight
 }

 // older versions of IE
 else {
   viewPortWidth = document.getElementsByTagName('body')[0].clientWidth,
   viewPortHeight = document.getElementsByTagName('body')[0].clientHeight
 }
 return [viewPortWidth, viewPortHeight];
}

( http://andylangton.co.uk/articles/javascript/get-viewport-size-javascript/ )

However, it is not even possible to get the viewport information in all browsers (e.g. IE6 in quirks mode). But the above script should do a good job :-)

You may use shorter version:

<script type="text/javascript">
<!--
function getViewportSize(){
    var e = window;
    var a = 'inner';
    if (!('innerWidth' in window)){
        a = 'client';
        e = document.documentElement || document.body;
    }
    return { width : e[ a+'Width' ] , height : e[ a+'Height' ] }
}
//-->
</script>

I've always just used document.documentElement.clientHeight/clientWidth. I don't think you need the OR conditions in this case.