<!DOCTYPE html>
<style>
#target {
    opacity: 0.5;
}
</style>
<div id="target" style="width: 500px; height: 500px; background-color: blue;"></div>
<div style="width: 300px; height: 300px; position: absolute; left: 20px; top: 400px; background-color: purple; z-index:1000">
    <div style="width:100px; height:100px; position:relative; left:10px; top:10px; transform:translateZ(0); background-color:yellow"></div>
</div>