1、只有块级box可以collapse;
2、floated box 的margins不会collapse;
3、overflow （而不是visible）不会同它的子元素发生collapse;
4、root element不collapse 指什么？
5、

If A‘s parent is ‘rl’:
If A‘s margins are collapsed with its parent‘s right margin, the right border edge of A is defined to be the same as the parent‘s.
Otherwise (i.e., either the element‘s parent is not taking part in the margin collapsing, or only the parent‘s left margin is involved), the position of A‘s right border edge is the same as it would have been if A had a nonzero top border。

？？