Rachel Andrew with a clear (get it?!) explanation of display: flow-root;, including demos comparing old and new techniques. Apparently the name is still a little bit still up in the air.
The whole point of it is getting rid of clearfix (although it is a bit different), or using a different/unintended property for float clearning. Every time this is brought up, there is always a well actually about how overflow: hidden; does the same (or any other … Read article
News to me! There is a spec for it and Firefox says they intend to ship it. It's just like display: block; only:
It always establishes a new block formatting context for its contents.
Meaning: you don't have to use clearfix hacks. There is a bit more to it though. Fiona Chan has a mini-talk all about block formatting context. I think it affects z-index and transforms in some cases too. Anybody?