Interview Question for Front End Engineer at Yelp:

When a div contains only floating elements it thinks nothing is inside so it will have a height of 0 if none is specified. This is known as a floating div clearing problem since the floating divs are not being cleared inside the parent div. There are a couple solutions, you can add an extra empty div after the floating divs with style clear:both to clear the floating divs. Only problem is this adds extra markup. Alternatively, there are some known CSS hacks such as setting the width:100% and the overflow:auto on the parent div.

overflow:hidden;