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

<style>
  * { position: relative }

  #overlay,
  #hidden {
    height: 50px;
    width: 50px;
  }

  #overlay {
    background: blue;

    z-index: 1;
  }

  #hidden {
    background: red;
    top: -50px;

    z-index: -1;
  }
</style>

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