为什么scrollTop没有效果？设置的值为50，但是alert的值为0
<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>无缝滚动</title>
<style type="text/css" media="screen">
*{
margin: 0px;
padding: 0px;
}
#box{
width: 400px;
height: 230px;
position: absolute;
left: 200px;
top:200px;
border: 5px solid #808080;
box-shadow: 5px #CBCBCB;
border-radius: 10px;
overflow:hidden;
}
#titlename{
width: 400px;
height: 80px;
border: 1px solid #000;
background-color: #dc143c;
}
#name{
width: 100px;
height: 70px;
border:1px solid #000;
font-size: 40px;
color:#fff;
position: relative;
font-weight:bold;
left: 20px;
top:10px;
}
#more{
width: 40px;
height: 20px;
border:1px solid #000;
font-size: 15px;
color:#fff;
position: relative;
font-weight:bold;
left: 175px;
top:30px;
}
#list{
width: 398px;
height: 265px;
border:1px solid #000;
position: relative;
top:5px;
overflow: hidden;
}
#list li{
width: 380px;
height: 23px;
border: 1px solid #000;
margin-bottom: 5px;
list-style-type: none;
text-align: left;
position: relative;
left: 10px;
font-size: 17px;
}
#list li span{
width: 70px;
height: 13px;
border: 1px solid #000;
position: absolute;
right: 13px;
top:3px;
font-size: 3px;
color:#a9a9a9;
}
</style>
</head>
<body>
<div id="box">
<div id="title">
<div id="titlename"><span id="name">最新课程</span><span id="more">更多>></span></div>
</div>
<div id="box_body">
<ul id="list">
<li>1.学会HTML5绝对的屌丝逆袭<span>2013-09-18</span></li>
<li>2.tab页面切换效果（案例）<span>2013-10-09</span></li>
<li>3.圆角水晶按钮制作（案例）<span>2013-10-21</span></li>
<li>4.HTML+CSS基础课程(系列)<span>2013-11-01</span></li>
<li>5.分页页码制作（案例）<span>2013-11-06</span></li>
<li>6.导航条菜单的制作（案例）<span>2013-11-08</span></li>
<li>7.信息列表制作（案例）<span>2013-11-15</span></li>
<li>8.下拉菜单制作（案例）<span>2013-11-22</span></li>
<li>9.如何实现“新手引导”效果<span>2013-12-06</span></li>
</ul>
</div>
</div>
</body>
<script type="text/javascript" charset="utf-8" async defer>
window.onload=function(){
var box_body=document.getElementById("box_body");
box_body.scrollTop=50;
alert(box_body.scrollTop);
}
</script>
</html>
