<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>css_test</title>
<style type="text/css">
body{
    margin:0px;
    position: relative;
    left:0px;
    top:0px;
}
#a {  
    width: 200px;  
    height: 200px;    
    background: #ddd;  
    position: absolute;
    left:0px;
    top:0px;
    z-index: 0;  /*  零是默认值，也可以不写该行  */
}  
#b {  
    width: 200px;  
    height: 200px;    
    background: #8daac3;  
    position: absolute;
    left:50px;
    top:50px;  
    z-index: -2;  
}  
#c {  
    width: 200px;  
    height: 200px;  
    background: #ba945d;  
    position: absolute;
    left:100px;
    top:100px;  
    z-index: 1;  
}
</style>
</head>
<body>
<div id="a"></div>
<div id="b"></div>
<div id="c"></div>
</body>
</html>