<!doctype html>
<head>
<style>
.margin { margin:20px 0 }
p { margin:0 }
span { clear:left; float:left; width:20px; height:20px; background-color:green }
</style>
<body>
The two green floats should be stacked right on top of one another.
<div>
<span></span>
<span></span>
</div>