lisa.royal

It's a thing with floats. When you float the black boxes it takes them out of the 'flow' of the html and the parent wrapper is no longer effected by them. Float the parent wrapper as well and it will contain the black box children, or add an empty, not floated, clearing div at the end of your black boxes
<div style="clear: both;"></div> or there are other clever ways with clearfix classes.
 
Although all that is going to become redundant with flexbox soon