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