establish a new block formatting context

Any block element can establish a new block formatting context if it has one of the
following property/value pairs set on it:

float: left
float: right
position: absolute
display: inline-block
display: inline-table
display: table-cell
display: table
overflow: auto
overflow: scroll
overflow: hidden

参考：
http://www.communitymx.com/content/article.cfm?page=1&cid=6BC9D
http://www.yuiblog.com/blog/2010/05/19/css-101-block-formatting-contexts/