z-index经常被赋正值，但它其实可以为负值的，ie和ff都兼容。写特殊效果时可以试试。
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>阿当制作</title>
</head>
<style type="text/css">
.head{height:200px;background:#000;margin-left:50px;}
.layer{100px;height:100px;background:green;position:absolute;left:0;top:0;z-index:-2;}
.layer2{100px;height:100px;background:yellow;position:absolute;left:20px;top:50px;z-index:-1;}
</style>
<body>
<div class="head"></div>
<div class="layer"></div>
<div class="layer2"></div>
</body>
</html>
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