css-Sub Div set margin-top, the parent Div moves down with the child div

According to Graceie's blog

The reason this problem occurred is thataccording to the specification, if a box does not have a filler (padding-top) and a top border (border-top), then the top margin of the box overlaps with the top margin of the first child in its internal document flow.

Again the white point is: The top margin of the first child element of the parent element margin-top if a valid border or padding is not touched. The trouble of" leading "(parent element, ancestor Element) is constantly being found on a layer. As long as the leadership to set an effective border or padding can effectively control the goal of the non-leader of the margin to prevent it leapfrog, false preach Imperial decree, their own margin when the leader of the margin execution.

So the solution: set Border-top or Padding-top to the parent div.

http://www.hicss.net/do-not-tell-me-you-understand-margin/

css-Sub Div set margin-top, the parent Div moves down with the child div