<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<html lang="en">
 <head>
  <title>Margin Collapsing: top margins</title>
  <style type="text/css">
   .a { margin: 0; background: red; }
   .b { margin: 2em 0 0 0; background: red; }
   .c { margin: 1em 0 0 0; background: red; }
   .d { margin: 0; background: green; color: white; }
  </style>
 </head>
 <body>
  <p>There should be no red below.</p>
  <div class="a">
   <div class="b">
    <div class="c">
     <div class="d">
      This line should be green.
     </div>
    </div>
   </div>
  </div>
 </body>
</html>