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