You should add a sharper div before the closing containers:
<div style="clear:both;"></div>
Floating elements do not affect the height of the container ... coz it floats;).
Live: http://jsfiddle.net/LBH5h/
:
<div id="content">
<div style="float:left;"></div>
<div style="clear:both;"></div>
</div>