這樣寫為什么hover沒效果，只有點擊了才有效果

<!DOCTYPE?html?PUBLIC?"-//W3C//DTD?XHTML?1.0?Transitional//EN"?"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html?xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta?http-equiv="Content-Type"?content="text/html;?charset=utf-8"?/>
<title>導航菜單</title>
<style?type="text/css">
*{margin:0;?padding:0;?font-size:14px;}
a{color:#333;text-decoration:none}
.nav{list-style:none;?height:30px;?border-bottom:5px?solid?#F60;?margin-top:20px;?padding-left:50px;}
.nav?li{float:left}
.nav?li?a{display:block;?height:30px;text-align:center;?line-height:30px;?width:120px;?border-radius:12px?12px?0?0;?margin-left:1px;background-color:#ddd;}

.nav?li?a.on?,nav?li?a:hover{color:#fff;background-color:#F60;height:40px;?line-height:40px;??margin-top:-10px;}

</style>

</head>
<body>

<ul?class="nav"?id="nav">
????<li><a?class="on"?href="javascript:void(0);">首　　頁</a></li>
????<li><a?href="javascript:void(0);">新聞快訊</a></li>
????<li><a?href="javascript:void(0);">產(chǎn)品展示</a></li>
????<li><a?href="javascript:void(0);">售后服務</a></li>
????<li><a?href="javascript:void(0);">聯(lián)系我們</a></li>
</ul>
<script>
????var?nav?=?document.getElementById('nav');
????var?navList?=?nav.getElementsByTagName('a');
????for(var?i?=?0;i?<?navList.length;?i++){
????????navList[i].onclick?=?function(){
????????????for(var?j?=?0;j?<?navList.length;?j++){
????????????????navList[j].className?=?'';
????????????}
????????????this.className?=?'on'
????????}
????}
</script>
</body>
</html>