 rel="help" href="http://www.w3.org/TR/CSS21/zindex.html#painting-order"/>
  <style type="text/css">* { margin: 0; padding: 0 }
   .background { position: absolute; height: 100px; width: 300px; background: red; z-index: 0; }
   .negative { position: absolute; height: 150px; width: 400px; background: green; z-index: -1; }
  </style>
 </head>
 <body>
  <p>There must be a green box below and no red.</p>
  <div class="background"> <div class="negative"> </div> </div>
 </body>
</html>
