官方文档对‘overflow’属性的描述：
This property specifies whether content of a block container element is clipped when it overflows the element’s box. It affects the clipping of all of the element’s content except any descendant elements (and their respective contentand descendants) whose containing block is the viewport or an ancestor of the element
原文：http://www.cnblogs.com/JosephLiao/p/4275157.html