                        课程
                    
                        /前端开发
                        
                            /JavaScript
                        
                        /展开与收起效果

为何本节代码在360浏览器中预览会有卡顿现象，而在谷歌浏览器中就能平滑滚动呢？

<!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>自动展开</title>
    <style type="text/css">
        body
        {
            margin: 0 auto;
            padding: 0;
        }
        a:focus
        {
            outline: none;
        }
        
        #pn
        {
            background: #e8e8e8;
            height: 120px;
            width: auto;
            display: block;
            margin: 0 auto;
            padding: 5px;
            text-align: center;
        	overflow:hidden;
        }
        .slide
        {
            padding: 10px;
            width: 960px;
            margin: 0 auto;
            background: gray;
            text-align: center;
            height: 40px;
            line-height: 40px;
            color: #fff;
        }
    </style>
    <script type="text/javascript">
       
    
        
        //增加高度函数addH()
        var h = 0;    
        function addh() {  
           if(h<300){
               h += 5;
               document.getElementById('pn').style.height=h +'px';
           }else{
               return;
           }
		setTimeout('addh()',30);
        }
        
       //网页加载完毕时，调用增加高度函数addH()，等待5秒钟后调用减少高度函数subh()。 
        window.onload=function(){
            addh();
            setTimeout('subh()',3000);
        }
      
      
      // 减少高度函数subH()
    
        function subh() {
            if(h>0){
                h -= 5;
                document.getElementById('pn').style.height=h +'px';
            }else{
                document.getElementById('pn').style.display='none';
                return;
            }
            setTimeout('subh()',30);
        } 
    </script>
</head>
<body>
    <div id="pn">
        <h1>
            欢迎来到慕课网</h1>
        <h2>
            大幅广告</h2>
    </div>
    <p class="slide">
        慕课网欢迎你</p>
</body>
</html>