<script type="text/javascript">
function ShowSub(li) {//函數(shù)定義
var subMenu = li.getElementsByTagName("ul")[0] ;//獲取
subMenu.style.display = " block ";
}
function HideSub(li) {
var subMenu = li.getElementsByTagName("ul")[0];
subMenu.style.display = " none ";
}
</script>

*{ margin:0px; padding:0px;}
body{ font-family:Verdana, Geneva, sans-serif; font-size:14px;}
#nav{ width:600px; height:40px; background-color:#eee; margin:0 auto;}
ul{ list-style:none;}
ul li{ float:left; line-height:40px; text-align:center; width:100px;}//ie7顯示寬度的兼容性 設(shè)置width 而不用padding的自適應(yīng)
a{ text-decoration:none; color:#000; display:block;}
a:hover{ color:#F00; background-color:#666;}
ul li ul li{ float:none;background-color:#eee; margin:2px 0px;}
ul li ul{ display:none;}

<div id="nav">
<ul>
<li><a href="#">首頁</a></li>
<li onmouseover="ShowSub(this)" onmouseout="HideSub(this)" ><a href="#">課程大廳</a>
<ul>
<li><a href="#">JavaScript</a></li>
<li><a href="#">Html/CSS</a></li>
</ul> 
</li>
<li onmouseover="ShowSub(this)" onmouseout="HideSub(this)" ><a href="#">學習中心</a>
<ul>
<li><a href="#">視頻學習</a></li>
<li><a href="#">實例練習</a></li>
<li><a href="#">問與答</a></li>
</ul> 
</li>
<li><a href="#">經(jīng)典案例</a></li>
<li><a href="#">關(guān)于我們</a></li>
</ul>
</div>