<!DOCTYPE html>
<!--
 | Any computed value other than "none" for the "transform" results in
 | the creation of a stacking context.
 |   https://www.w3.org/TR/css3-transforms/#transform-property
 -->
<html>
<head>
  <style>
    .block {
      height: 100px;
      left: 20px;
      position: absolute;
      top: 20px;
      width: 100px;
    }
    .transformed {
      transform: translateY(150px) rotate(15deg);
    }
    .outer {
      background-color: #0091ea;
    }
    .inner {
      background-color: #00b0ff;
      z-index: -1;
    }
  </style>
</head>
<body>
  <div class="outer block">
    <div class="inner block"></div>
  </div>
  <div class="transformed outer block">
    <div class="inner block"></div>
  </div>
</body>
</html>