Why does overflow hidden stop floating elements escaping their container?

Question

A common problem that I have with web pages is floating div tags creeping outside of their containers, like shown in the snippet.

#wrapper {   border: 1px solid red; }  #wrapper div {   float: left;   font-size: 3em; }

<div id="wrapper">   <div>Hello World</div> </div>

There are a lot of dirty ways to fix this (i.e., inserting a div with clear:both)

However, a much neater solution I have seen is setting the wrapper div with overflow set to hidden:

#wrapper {   border: 1px solid red;   overflow: hidden; }  #wrapper div {   float: left;   font-size: 3em; }

<div id="wrapper">   <div>Hello World</div> </div>

This works well across browsers, nice and cleanly with no additional markup. I am happy, but I have no idea WHY it works this way?

All the documentation, I had looked at, indicates overflow:hidden is for hiding content, not resizing a parent to fit its children...

Can anybody offer a explanation for this behavior?

Thanks

Original snippets: Live example 1: http://jsfiddle.net/ugUVa/1/ Live example 2: http://jsfiddle.net/ugUVa/2/

Answer 1

It creates a block formatting context.

Block formatting contexts are important for the positioning (see float) and clearing (see clear) of floats. The rules for positioning and clearing of floats apply only to things within the same block formatting context. Floats do not affect the layout of things in other block formatting contexts, and clear only clears past floats in the same block formatting context.

see also: http://www.w3.org/TR/CSS2/visuren.html#block-formatting