<!DOCTYPE html>
<meta charset="utf-8" />
<title>Element hidden by other element</title>

<style>
  div {
    position: absolute;
    height: 100px;
    width: 100px;
  }

  #overlay {
    background: blue;
  }

  #hidden { background: red }
</style>

<div id="hidden"></div>
<div id="overlay"></div>