Q:

When child divs are floated, parent divs lose auto-height

I have a parent div called main.  It has a bunch of child divs in it, which I floated left or right. 
When I finished floating the last one, the height of the main div completely vanished, throwing off the layout of my page.  I gave it a height:100%; just for now, but it will be overly tall on most resolutions.  How can I keep it at a fluid height, only as much as needed to contain the elements? 

A:

Contain your floats. 
On the div called main add overflow:hidden;.   That should contain it.