<!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: 0; background: red; }
.d { margin: 1em 0 0 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>