number z-index:3; 按照z-index的大小排序
因为Z-index只能工作在被明确定义了absolute，fixed或relative 这三个定位属性的元素中
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
<style type="text/css">
.blue,.green,.red{
width:200px;
height:200px;
position:relative;
}
.blue{
background:blue;
z-index:3;
}
.green{
background:green;
margin-top:-100px;
margin-left:50px;
z-index:2;
}
.red{
background:red;
margin-top:-100px;
margin-left:100px;
z-index:1;
}
</style>
</head>
<body>
<div class="blue"></div>
<div class="green"></div>
<div class="red"></div>
</body>
</html>
