广告的字一个一个的显示出来

<!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=gb2312" />
<title>无标题文档</title>
<script>

var str = "这是一个在线拍卖的网站，请尽情血拼吧！";
var seq=0;
function scroll()
{
msg=str.substring(0,seq+1);
banner.innerHTML=msg;
seq++;
if(seq >= str.length) seq =0;

}

</script>
</head>