<style type="text/css">
#test {
height: 200px;
width: 200px;
background-color:red;
}
</style>
<script>
window.onload = function(){
var div = document.getElementById("test");
var s = div.currentStyle || window.getComputedStyle(div,'') || document.defaultView.getComputedStyle(div, '');
var p = s.width || s.getPropertyValue("width");
alert(p);
</script>