<!DOCTYPE html>
<html>
<head>
<style type="text/css">
#separator {
 height: 20px;
 background-color: green;
}
#margin {
 height: 40px;
}
#float {
 height: 20px; width: 50px;
 margin-right: auto;
 background-color: lightgreen;
}
</style>
</head>
<body>
<div id="separator"></div>
<div id="margin"></div>
<div id="float"></div>
</body>
</html>