[UnderShot]

<style type="text/css">
div {
    background: blue;
    position: absolute;
    color: white;
    text-align: center;
    width: 100px;
    height: 100px;
}
</style>

<script type="text/javascript">
window.onload = function(){
    var i = 0;
    (function(){
        if(++i <= 300){
           var element = document.getElementById('qwe');
           element.style.width = i+'px';
           element.innerHTML = parseInt(i/3)+'%';
           setTimeout(arguments.callee, 10);
        }
    })();
};
</script>
<div id="qwe"></div>