<!DOCTYPE html>
<html>
<head>
<style type="text/css">
.separator {
 height: 20px;
 background-color: green;
}
#parent {
 background-color: lime;
 height: 100px;
}
#float {
 width: 100px; height: 100px;
 background-color: blue;
 margin-right: auto;
}
</style>
</head>
<body>
<div class="separator"></div>
<div id="parent">
 <div id="float"></div>
</div>
<div class="separator"></div>
</body>
</html>