Clearfix.
when an element only contains floated elements, it collapses on itself. To prevent this behavior, you have to “clearfix” it. We used to do it with an extra element, but not anymore.

Note: The space content is one way to avoid an Opera bug when the contenteditable attribute is included anywhere else in the document. Otherwise it causes space to appear at the top and bottom of elements that are clearfixed.

Note: The use of display: table rather than display: block is only necessary if using the :before pseudo-element to contain the top-margins of child elements.