在ie中他的子类的zindex就以父类为准；
<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>无标题文档</title>
<style>
#div1{ width:200px; height:200px; float:left; border:1px solid red;}
#div2{ width:200px; height:200px; float:left; border:1px solid black;}
.blue{ width:300px; height:300px; background-color:blue;}
.green{ width:300px; height:300px; background-color:green;}
</style>
</head>
<body>
<div id="div1" style="position:relative;z-index:1;">
<div class="blue" style="position:absolute;z-index:9999"><span style="color:red;">zIndex:9999</span></div>
</div>
<div id="div2" style="position:relative;z-index:2;">
<div class="green" style="position:absolute;z-index:10"><span style="color:#fff">zIndex:10</span></div>
</div>
</body>
</html>
div1子类的zindex为9999，父类的zindex为1， div2的子类的zindex为10，父类的zindex为2，比较父类的zindex大小，所以div1，无法覆盖div2