ie6 z-index bug 的解决方案
1 把浮动在最上面的元素放在html代码的最后面
2 以下是正解
giving the parent element a higher z-index actual fixes the bug
http://brenelz.com/blog/squish-the-internet-explorer-z-index-bug/
原因:“In Internet Explorer positioned elements generate a new stacking context, starting with a z-index value of 0. Therefore z-index doesn’t work correctly”