Why does the child element margin-top affect the parent element?

The problem is
the following very simple code:

css is as follows:

<style type="text/css">
        *{
            margin: 0px; padding: 0px; 
        }
        .show{
            margin: 0px auto;
            width: 200px;
            height: 100px;
            background-color: #999999;
        }
        .show h2{
            margin-top: 50px;/*important*/
            cursor: pointer;
        }
    </style>

The html structure is as follows

<div class="show">
        <h2>crystal</h2>
    </div>

When <h2>crystal</h2>no settings margin-topare made, the browser displays as expected
这里写图片描述

After setting margin-top, the following phenomenon appears:
这里写图片描述

We did not set margin-top for the outer div, but there is still a 50px gap from the top of the browser. Check which element the 50px gap belongs to, and you will find that the margin of the outer div is still 0 auto; so this margin is still belongs to h2, why is this?

Look at the content specified in the css2.1 box model:

In this specification, the expression collapsing margins means that adjoining margins (no non-empty content, padding or border areas or clearance separate them) of two or more boxes (which may be next to one another or nested) combine to form a single margin . The margins of all adjacent two or more box elements will be merged into one shared margin. Adjacency is defined as: sibling or nested box elements with no non-empty content, padding, or border between them.

Because nesting is also adjacent, the higher-priority margin in the style sheet overrides the.show h2 margin defined by the previous outer div, resulting in a 10px spacing for the entire div.
If you also add a class to h2 and .showdefine it before, the final result is as shown in the first figure, and the final margin is displayed as 0;

解决办法：

1. 父级或子元素使用浮动或者绝对定位absolute

浮动或绝对定位不参与margin的折叠

2. 父级overflow:hidden;

3. 父级设置padding（破坏非空白的折叠条件）

4. 父级设置border

资料多数参考如下文章：http://www.cnblogs.com/hejia/archive/2013/05/26/3099697.html

问题如下
一段很简单的代码：

css如下：

<style type="text/css">
        *{
            margin: 0px; padding: 0px; 
        }
        .show{
            margin: 0px auto;
            width: 200px;
            height: 100px;
            background-color: #999999;
        }
        .show h2{
            margin-top: 50px;/*important*/
            cursor: pointer;
        }
    </style>

html结构如下

<div class="show">
        <h2>crystal</h2>
    </div>

当在<h2>crystal</h2>没有设置margin-top时，浏览器显示如预期
这里写图片描述

设置了margin-top后出现了如下所示的现象：
这里写图片描述

我们并没有给外层的div设置margin-top，但是还是距离浏览器最顶部产生了50px的间距，查看这50px间距属于哪个元素，会发现外层div的margin依然是0 auto; 所以这个margin还是属于h2,为什么会这样？

来看css2.1盒模型中规定的内容：

In this specification, the expression collapsing margins means that adjoining margins (no non-empty content, padding or border areas or clearance separate them) of two or more boxes (which may be next to one another or nested) combine to form a single margin. 所有毗邻的两个或更多盒元素的margin将会合并为一个margin共享之。毗邻的定义为：同级或者嵌套的盒元素，并且它们之间没有非空内容、Padding或Border分隔。

因为嵌套也属于毗邻，所以在样式表中优先级更高的 .show h2的margin覆盖了之前外层div定义的margin，导致最终整个div产生10px的间距。
若给h2也添加一个class，并且在.show之前定义，则最终结果如第一个图所示，最终margin显示为0；