<!DOCTYPE html>
<script>
    window.onload = function() {
        document.body.offsetTop;
        document.getElementById('a').style.display = 'inline-block';
    }
</script>

<p>There should be a blue <em>square</em> below.</p>
<div>
    <div id="a" style="display:none; width:20px; height:40px; background:blue;"></div>
    <div style="float:left; width:20px; height:40px; background:blue;"></div>
    <div></div>
</div>