<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>无标题文档</title>
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs
/jquery/1.4.0/jquery.min.js"></script>
<style type="text/css">
.aa,.bb{width:200px; height:200px; margin:10px; background-color:#000}
.bb{background-color:#F00; opacity:0.5}
</style>
<script type="text/javascript">
$(function(){
$(".aa").css("opacity",0.1)
})
</script>
</head>
<body>
<div class="aa"></div>
<h2>JQ版</h2>
<div class="bb"></div>
<h2>CSS版</h2>
</body>
</html>