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

<style>
  * { position: relative }

  #overlay {
    height: 50px;
    width: 50px;
    background: blue;
    z-index: 1;
  }

  #hidden {
    height: 50px;
    width: 50px;
    background: red;
    z-index: -1;

    transform: translate(0, -50px);

    /* fix your browsers god damnit */
    -webkit-transform: translate(0, -50px);
    -moz-transform: translate(0x, -50px);
  }
</style>

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