bootstrap.css: .container:before display table

http://nicolasgallagher.com/micro-clearfix-hack/

The clearfix hack is a popular way to contain floats without resorting
to using presentational markup. This article presents an update to the
clearfix method that further reduces the amount of CSS required.

http://html5boilerplate.com/docs/The-style/#clearfix:

Adding .clearfix to an element will ensure that it always fully
contains its floated children. There have been many variants of the
clearfix hack over the years, and there are other hacks that can also
help you to contain floated children, but the H5BP currently provides
this micro clearfix helper class.

• .clearfix:before, .clearfix:after { content: ""; display: table; }
  This rule is understood by all browsers except for IE6/7. It generates
  a pseudo-element before and after the content of the element that
  contains floats. Setting display: table creates an anonymous
  table-cell and a new block formatting context. This acts to prevent
  top-margin collapse and improve the consistency between modern
  browsers and IE6/7.

• .clearfix:after { clear: both; }
  Makes the :after pseudo-element clear the floated children of this element,
  thereby making the element expand to contain the floats.

• .clearfix { zoom: 1; }
  Create new block formatting context in IE6/7 by triggering hasLayout