我在阅读CSS2.2，但是我真的无法理解这句话的意思，以及它所适合的场景：
If the top and bottom margins of an element with clearance p. 157 are adjoining, its margins collapse with the adjoining margins of following siblings but that resulting margin does not collapse with the bottom margin of the parent block.
StackOverflow上有个家伙也提了一样的问题，回答者的答案我也不理解。。。
借用采纳者的知乎链接，这句的意思是：
If the top and bottom margins of an element with clearance are adjoining, its margins collapse with the adjoining margins of following siblings but that resulting margin does not collapse with the bottom margin of the parent block.（有间隙不折叠)
哪，什么才是有间隙呢？这里的间隙clearance在css2.2中有定义。来个例子，先有个直观的理解吧：
<html>
<head>
<style>
* {padding: 0; margin: 0;}
#b1 {
width: 300px;
height: 100px;
margin-bottom: 4em;
background-color: #0d8ba1;
}
#F {
float: left;
width: 100px;
height: 70px;
background-color: yellow;
}
#b2 {
/*clear: both;*/
width: 300px;
height: 100px;
background-color: red;
margin-top: 2em;
}
</style>
</head>
<body>
<p id="b1"></p>
<p id="F"></p>
<p id="b2"></p>
</body>
</html>运行时，对比#b2有clear:both与没clear:both时，b1与b2margin的折叠情况。
设置clear:both之后，#b2为了避开浮动的F，往下移动，出现了clearance也就是空隙，b1与b2不再折叠！
另外，stack overflow那个家伙的答案我没再细究。pass。
怪我咯2017-04-17 11:39:17
This explanation is also quite clear:
http://www.raccoon-tech.com/css-margin-collapse
阿神2017-04-17 11:39:17
As long as you understand that the condition for clearance is that when the clear element is not cleared (normally), its boarder top is higher than the boarder bottom of the float element. If the clear element is originally under the float element, clearance will not occur.
If this hypothetical position of the element's top border edge is not past the relevant floats, then clearance is introduced, and margins collapse according to the rules in 8.3.1.
So this rule in plain language is: for an element with clearance, when it margin top and margin bottom are adjacent (for example, height is 0, there is no padding and border), these two The margin will be calculated together with the margin of the next adjacent element, but will not be calculated with the margin bottom of the parent element (in this case, only these two margins will be calculated).
Also note here that the clearance calculation is based on the boarder edge, so for the margin top and margin bottom collapse elements, you must finally note that they will also swallow up a height equivalent to margin top.
Then the amount of clearance is set to the greater of:
The amount necessary to place the border edge of the block even with the bottom outer edge of the lowest float that is to be cleared.
The amount necessary to place the top border edge of the block at its hypothetical position.
