compositing/geometry/foreground-layer.html

<!DOCTYPE html>
<html>
<head>
  <style type="text/css">
    .box {
      position: relative;
      display: inline-block;
      margin: 30px;
      width: 200px;
      height: 200px;
      -webkit-transform: translateZ(0);
    }

    .main {
      border: 40px solid black;
      box-shadow: black 0 0 20px; /* Makes compositing layer larger */
      background-color: red;
    }
    
    .child {
      position: absolute;
      width: 150px;
      height: 150px;
      padding: 20px;
      -webkit-transform: translateZ(1px);
    }
    
    .negative {
      z-index: -1;
      top: 0;
      left: 0;
      height: 10px;
      width: 10px;
    }

    img {
      background-color: green;
      height: 200px;
      width: 200px;
    }
    
    .embiggener {
      position: absolute;
      height: 10px;
      width: 10px;
      top: -50px;
      left: -50px;
      background-color: blue;
    }
    
    #layer-tree {
      opacity: 0; /* hide from pixel test */
      height: 10px; /* avoid scrollbars */
      overflow: hidden;
    }
    
  </style>
  <script type="text/javascript" charset="utf-8">
    if (window.testRunner)
      testRunner.dumpAsText(true);

    function dumpLayerTree()
    {
      if (window.testRunner)
        document.getElementById('layer-tree').innerText = testRunner.layerTreeAsText();
    }
    
    window.addEventListener('load', dumpLayerTree, false);
  </script>
</head>
<body>

<!-- Both boxes should look the same. You should see no red. -->
<div class="main box">
  <img>
  <div class="negative child"></div>
</div>

<div class="main box" style="overflow: hidden;">
  <img>
  <div class="negative child"></div>
</div>

<pre id="layer-tree"></pre>
</body>
</html>