It may seem counter-intuitive but overflow:hidden prevents margin collapse on parent elements containing floats:
The magic of “overflow: hidden” — Articles — Colin Aarts, freelance web developer
Until browser support for Flexbox improves, another alternative to floats is display:table-cell which has the added benefit of making equal height columns.
Alt-Web Templates :: Equal Height CSS Columns with display:table property
CSS provides many different ways to skin the proverbial cat. It's good to be familiar with all of them.
Nancy O.