<style>
    .parent {
        position: relative;
        background-color: black;
        width: 100px;
        height: 100px;
    }
    .child {
        position: relative;
        background-color: rgba(0,255,0,0.8);
        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;
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>