The floated elements escape from their parent overflow. As a result, their height is not added to the height of the parent. To bypass that, there are several methods. My favorite is inserting an empty DIV, as the child, to clear the floats:
Code:
<div id="parent">
<div style="float:left"></div>
<div style="float:right"></div>
<div style="clear:both"></div>
</div>