http://www.lvjiyong.com/en-us/item/How-To-Clear-Floats-Without-Structural-Markup When a float is contained within a container box that has a visible border or background, that float does not automatically force the container's bottom edge down as the float is made taller. Instead the float is ignored by the container and will hang down out of the container bottom like a flag. Those familiar only with Explorer for Windows may scratch their heads and say That's not right! True, IE/Win does enclose a float within a container 'automatically', but only if the container element has a stated dimension, and in any case it's a W3C spec violation. This illegal behavior can also be 'toggled' on and off by links within the container, if 'hovering' alters the link background or several other properties. Quite a mess. en-us Mon, 01 Jan 0001 00:00:00 GMT Mon, 01 Jan 0001 00:00:00 GMT 吕的部落格 Version 1.0.0.0