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<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="utf-8" />
    <title></title>
</head>
<style type="text/css">
    .top-nav {
        font-size: 14px;
        font-weight: bold;
        list-style: none;
    }
    .top-nav li {
        float: left;
        margin-left: 1px;
    }
    .top-nav li a {
        line-height: 34px;
        text-decoration: none;
        background: #3f240e;
        color: #fff;
        display: block;
        width: 80px;
        text-align: center;
    }
    .top-nav ul {
        list-style: none;
        display: none;
        padding: 0;
        position: absolute;
        height: 0;
        overflow: hidden;
    }    
    .top-nav li a:hover {
        background: url(images/slide-bg.png) 0 0 repeat-x;
    }
    .note {
        color: #3f240e;
        display: block;
        background: url(images/slide-bg.png) 0 0 repeat-x;
    }
    .corner {
        display: block;
        height: 11px;
        background: url(images/corner.png) 31px 0 no-repeat;
    }
</style>

<script type="text/javascript">
     window.onload = function() {
         var Lis = document.getElementsByTagName("li");
         for(var i=0; i<Lis.length; i++) {
             Lis[i].onmouseover = function() {
                 var u = this.getElementsByTagName("ul")[0];
                 if(u != undefined) {
                 u.style.display = "block";
                 AddH(u.id);    
                 }
                 
             }

             Lis[i].onmouseleave = function() {
                 var u = this.getElementsByTagName("ul")[0];
                 if(u != undefined) {
                 SubH(u.id);    
                 }
                 
             }
         }
     }
    function AddH(id) {
        var ulList = document.getElementById(id);
        var h = ulList.offsetHeight;
        h+=1;
        if(h<43) {
            ulList.style.height = h + "px";
            setTimeout("AddH(' "+id+"')",10);
        }
        else {
            return;
        }
        
    }

    function SubH(id) {
        //setTimeout();
    }
</script>

<body>
    <ul class="top-nav">
    <li><a href="#"><span class="note">慕課網(wǎng)</span></a></li>
    <li><a href="#">課堂大廳</a></li>
    <li><a href="#">學(xué)習(xí)中心</a>
    <ul id="mnuUL">
        <span class="corner"></span>
        <li><a href="#">前端課程</a></li>
        <li><a href="#">手機(jī)開發(fā)</a></li>
        <li><a href="#">后臺(tái)編程</a></li>
    </ul>
    </li>
    <li><a href="#">關(guān)于我們</a></li>
    </ul>
</body>
</html>

如果滑鼠在上面快速來回滑動(dòng)，動(dòng)畫才會(huì)慢慢出來
·TypeError: null is not an object (evaluating 'ulList.offsetHeight')·