auto，0和没有z-index之间的区别？
有什么区别：
What is the difference between:
- z-index：auto
- z-index ：0
- 完全没有z索引
以上所有情况均适用于div包含两个div，div1和div2，每个div的索引分别为9和10.
all the above scenarios are for a div that encloses two divs, div1 and div2 each having a z index which is 9 and 10 respectively.
封闭div位于html的堆叠环境中。
The enclosing div is in the stacking context of html.
不指定 z-index 与 z-index相同：auto ;这是它的初始值。
Not specifying z-index is the same as z-index: auto; that is its initial value.
auto 和 0 如果你的元素没有创建自己的堆栈上下文，这意味着同样的事情;例如它不是相对的，绝对的或固定的。
auto and 0 mean the same thing if your element doesn't create its own stacking context; e.g. it is not positioned as relative, absolute or fixed.
如果您的封闭 div 未定位，你将它的 z-index 设置为无关紧要;它及其所有内容将参与 html 的堆叠环境，并且其后代将始终位于其前面。
If your enclosing div isn't positioned, then whatever you set its z-index to doesn't matter; it and all its contents will participate in the stacking context of html, and its descendants will always be positioned in front of it.