http://jsfiddle.net/gothic/B6E8F
It does not mean that the vertical margins of two or more adjacent block elements in ordinary flows will collapse. So which condition does this example violate?
曾经蜡笔没有小新2017-05-19 10:27:45
w3.org Specification on margin merging
Two margins are adjoining if and only if:
both belong to in-flow block-level boxes that participate in the same block formatting context
no line boxes, no clearance, no padding and no border separate them
...
Margin needs to belong to the block-level box in the same BFC, overflow: hidden A new BFC is created, so it will not collapse naturally.