overflow

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.

.group {
  display: flow-root;
}

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?

I suspect Nicole Sullivan will approve. She's been writing and talking about new formatting contexts forever. In 2010 she even wrote:

I wish we had a simple property value pair that would do the same thing in standards-based browsers.

.lastUnit {
  formatting-context: new; /* please! */
}

Just using overflow isn't a real solution. It hides box-shadow, for one thing.

Also See

• Rachel Andrew: The End of the Clearfix Hack?
• Thierry Koblentz: display: flow-root versus clearfix