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