<!DOCTYPE html>
<html>
<head>
<meta charset=" utf-8">
<title>蚂蚁部落</title>
<style type="text/css">
#box{
width:300px;
height:200px;
background:black;
}
#middle{
width:200px;
height:150px;
background:red;
margin:0px auto;
}
#inner{
width:100px;
height:100px;
background:blue;
margin:0px auto;
}
</style>
<script src="http://libs.baidu.com/jquery/1.9.0/jquery.js"></script>
<script>
$(document).ready(function(){
$("#box>div").css("background-color","green");
});
</script>
</head>
<body>
<div id="box">
<div id="middle">
<div id="inner"></div>
</div>
</div>
</body>
</html>