<html>
<head>
<style>
.unchanged + #test
{
  background-color:red
}

.changed + #test
{
  background-color:green;
}

#test {
  width:100px;
  height:100px;
}
</style>
</head>
<body>
There should be a 100x100 green block below.

<div class="unchanged" id="beforetest"></div>

<div id="test"></div>
<script>
document.body.offsetWidth;
document.getElementById('beforetest').className = 'changed';
document.body.offsetWidth;
</script>
</body>
</html>