<!DOCTYPE html>
<html>
<head>
<style type="text/css">
#a {
background-color: green;
margin-bottom: 10px;
}
#a div {
height: 20px;
margin-bottom: 40px;
#b {
margin-top: 20px;
</style>
</head>
<body>
<div id="a">
<div></div>
</div>
<div id="b">
</body>
</html>