<!DOCTYPE html>
<!--
 | "z-index" applies to positioned elements.
 |   https://www.w3.org/TR/CSS21/visuren.html#z-index
 -->
<html>
<head>
  <style>
    .outer {
      border: 5px solid #00b0ff;
      background-color: #0091ea;
      height: 100px;
      width: 100px;
      z-index: 0;
    }
    .inner {
      border: 5px solid #00e676;
      background-color: #00c853;
      height: 100px;
      left: 50px;
      position: absolute;
      top: 50px;
      width: 100px;
      z-index: -1;
    }
  </style>
</head>
<body>
  <div class="outer">
    <div class="inner"></div>
  </div>
</body>
</html>