<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN">
<html lang="en">
 <head>
  <title>Margin Collapsing: negative margins (easy)</title>
  <style type="text/css">
   .container { background: red; height: auto; border-top: 1em lime solid; border-bottom: 1em lime solid; }
   .red { border-top: 1em solid red; }
   .lime { border-top: 1em solid lime; margin-top: -1em; }
  </style>
 </head>
 <body>
  <p>There should be no red below.</p>
  <div class="container">
   <div class="red"></div>
   <div class="lime"></div>
  </div>
 </body>
</html>