<style>
.parent {
    position: relative;
    background-color: black;
    width: 100px;
    height: 100px;
}
.child {
    background-color: red;
    width: 100px;
    height: 100px;
}
</style>
<div class=parent>
    <div class=child>
    </div>
</div>
<div class=parent>
    <div>
        <div class=child>
        </div>
    </div>
</div>
<script>
document.body.offsetTop;
if (window.testRunner)
    testRunner.displayAndTrackRepaints();
var children = document.getElementsByClassName("child");
children[0].style.backgroundColor = "rgba(0,255,0,0.5)";
children[1].style.backgroundColor = "rgba(0,255,0,0.5)";
</script>