<!DOCTYPE html>
<p>On this page there should be a blue square above a yellow square.</p>
<div style="position:relative; width:500px;">
    <div id="elm" style="margin:auto; width:100px; height:400px; background:yellow;">
        <div></div>
        <div style="position:absolute; width:200px; height:200px; background:blue;"></div>
    </div>
</div>
<script>
    document.body.offsetTop;
    document.getElementById("elm").style.width = "200px";
</script>