<div id="target" style="width: 200px; height: 100px; background-color: red;">
    <div style="width: 100px; background-color: green;">
        <span style="float: left; width: 10px; height: 100px;">&nbsp;</span>
        <br style="clear: both;">
    </div>
</div>
<script>
    document.body.offsetTop;
    document.getElementById("target").style.width = "100px";
</script>