<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>z-index</title>
<link href="css/index.css" rel="stylesheet" type="text/css" />
</head>
<body>
<div id="block1">第一个div</div>
<div id="block2">第二个div</div>
<div id="block3">第三个div</div>
</body>
css样式：
@charset "utf-8"; /* CSS Document */
div { 200px; height:60px; position:absolute; border:1px dashed #0000ff; }
#block1 { left:20px; top:20px; z-index:1; background-color:#99ccff; }
#block2 { left:80px; top:40px; z-index:3; background-color:#ffff66; }
#block3 { left:60px; top:60px; z-index:-1; background-color:#ff9900; }