1. 某些浏览器下元素层级遮盖存在bug;

2. 某个元素z-index设的太大，导致始终无法被遮盖;

3. js动态计算z-index，导致元素覆盖关系部可控

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可以这样理解：定位树中包含了dom树中position不为staitc的全部元素。非同级元素比较层级关系可以这样比较：

1. 向上遍历定位树的父节点直到2个元素为同级元素。

2. 根据上面的结论来最最后的比较。层级高的元素会越靠近用户的显示器并能覆盖层级低的元素。


实例一：假设上图中所有标红色元素position都不为static;且z-index=1，根据上面的比较规则可以知道：

1. c > (b && b *) > (a && a *) 即元素c及c的所有定位元素层级比a、b高

2. m > h、 g > k、d > m

实例二：假设a的position不为static，z-index属性不存在，定位树中a的子元素z-index一次分别为1,2,3,4...其它定位元素z-index:1：

1. 根据结论三有：IE6/7下 b > (a && a *) 即元素b的层级比a及a的所有定位元素层级高，其它浏览器下：i > h > b > d