<div>
    <p> The black box should be 5px below the float. </p>
    <div style="height: 20px; width:50px; background-color: green; margin-bottom: 5px;"></div>
    <div style="height: 10px; width:50px; background-color: blue; float:left;"></div>
    <div style="height: 20px; width:50px; background-color: black; margin-top: 20px; clear:both"></div>
</div>