.clearfix
is defined in less/mixins.less
. Right above its definition is a comment with a link to this article:
The article explains how it all works.
UPDATE: Yes, link-only answers are bad. I knew this even at the time that I posted this answer, but I didn't feel like copying and pasting was OK due to copyright, plagiarism, and what have you. However, I now feel like it's OK since I have linked to the original article. I should also mention the author's name, though, for credit: Nicolas Gallagher. Here is the meat of the article (note that "Thierry’s method" is referring to Thierry Koblentz’s “clearfix reloaded”):
This “micro clearfix” generates pseudo-elements and sets their
display
totable
. This creates an anonymous table-cell and a new block formatting context that means the:before
pseudo-element prevents top-margin collapse. The:after
pseudo-element is used to clear the floats. As a result, there is no need to hide any generated content and the total amount of code needed is reduced.Including the
:before
selector is not necessary to clear the floats, but it prevents top-margins from collapsing in modern browsers. This has two benefits:
It ensures visual consistency with other float containment techniques that create a new block formatting context, e.g.,
overflow:hidden
It ensures visual consistency with IE 6/7 when
zoom:1
is applied.N.B.: There are circumstances in which IE 6/7 will not contain the bottom margins of floats within a new block formatting context. Further details can be found here: Better float containment in IE using CSS expressions.
The use of
content:" "
(note the space in the content string) avoids an Opera bug that creates space around clearfixed elements if thecontenteditable
attribute is also present somewhere in the HTML. Thanks to Sergio Cerrutti for spotting this fix. An alternative fix is to usefont:0/0 a
.Legacy Firefox
Firefox < 3.5 will benefit from using Thierry’s method with the addition of
visibility:hidden
to hide the inserted character. This is because legacy versions of Firefox needcontent:"."
to avoid extra space appearing between thebody
and its first child element, in certain circumstances (e.g., jsfiddle.net/necolas/K538S/.)Alternative float-containment methods that create a new block formatting context, such as applying
overflow:hidden
ordisplay:inline-block
to the container element, will also avoid this behaviour in legacy versions of Firefox.