你可以从Github或从http://twitter.github.com/bootstrap/#找到
为什么它使用：
.container:before,.container:after {
display: table;
content: "";
zoom: 1;
}
.row:before,.row:after {
display: table;
content: "";
zoom: 1;
}
为什么要定义display：table in .container：before / after和.row：before / after？
解决方法
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
.clearfixto 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. Settingdisplay: tablecreates an anonymous
table-celland 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:afterpseudo-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 triggeringhasLayout