请问我这是错在哪里 ？？快哭了= =
<!DOCTYPE html>
<html>
<head>
<title>浏览器对象</title>
<meta http-equiv="Content-Type" content="text/html; charset=gkb"/>
</head>
<body>
<!--先编写好网页布局-->
<h1>操作成功</h1>
<p>
<span id="sec"><strong>5</strong></span>秒后成功<a href="history.back">返回</a>
</p>
<script type="text/javascript">
var timer=document.getElementById("sec").innerHTML;
function change(){
timer--;
document.getElementById("sec").innerHTML=timer;
if(timer==0){
window.location.assign("http://www.imooc.com");
}
}
setInterval("change()",1000);
</script>
</body>
</html>
