/* put them into the same context */
z-index: 2;
background-color: #f00;
}
二、创建stack context及注意事项
那么创建stack context的方式有哪些？
1) When an element is the root element of a document (theelement)
2) When an element has a position value other than static and a z-index value other than auto
3) When an element has an opacity value less than 1
Update: In addition to opacity, several newer CSS properties also create stacking contexts. These include: transforms, filters, css-regions, paged media, and possibly others. As a general rule, it seems that if a CSS property requires rendering in an offscreen context, it must create a new stacking context.
In WebKit, styling a box with position:fixed or -webkit-overflow-scrolling:touch implicitly creates a stacking context, just like adding a z-index value.
Also, be aware of these CSS3 “triggers”:
transform != none
transform-style: preserve-3d
filter != none
clip-path, mask
Lastly, even though a relatively positioned element without a z-index set does not establish a stacking context…
A common IE bug, often seen in drop-down menus, is that any relatively positioned element that has haslayout set to true establishes a stacking context.
One may visualize this bug by setting [A] and [B] to position:relative, while [a] gets position:relative; z-index:1.
Now, dragging [A] under [B] hides [a] – in Internet Explorer, that is. Any positioned child with a z-index is caught by this wrong stacking context of its parent.
三、z-index在某些浏览器中的问题
1) IE6中的
select元素是一个窗口控件，所以它总是出现在层叠顺序的顶部而不会顾及到自然层叠顺序、position属性或者是z-index。可以在div元素上添加一个iframe设置为position:absolute，并设置div的z-index比iframe的高。
2) 因父容器（元素）被定位的缘故，IE6/7会错误的对其stacking context进行重置。
3) 在Firefox2版本中，一个负的z-index值会使元素位于stacking context的后面，而不是位于公认的背景和边框这样的元素stacking context之前。
本文到此结束，最后附上本文开始时提出的问题的答案：
复制代码
/* add this */
div:first-child {
opacity: .99;
}
感谢您的阅读，文中不妥之处，还望批评指正。
