原理是这样的，外围元素之所以不能很好的延伸，问题出在了overflow上，因为overflow不可见。见W3C的解释
引用内容：
In addition, if the element has any floating descendants whose bottom margin edge is below the bottom, then the height is increased to include those edges. Only floats that are children of the element itself or of descendants in the normal flow are taken into account, e.g., floats inside absolutely positioned descendants or other floats are not. 现在只要将给外围元素添加一个overflow:auto;就可以解决问题，结果是除了IE，真的可以解决。下来就要解决ie的问题了，再加上_height:1%; ，这个问题就完全解决了。