打开网页后定时显示大图片广告然后自动关闭

网页特效代码实例,打开网页后定时显示大图片广告然后自动关闭,一些门户比如163首页就有这种广告效果.

以下是代码片段：
<!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>
    <title>顶部展出的大幅广告</title>
   <script type="text/javascript" language="javascript">
    var time = 500;
    var h = 0;
    function addCount()
    {
        if(time>0)
        {
            time--;
            h = h+5;
        }
        else
        {
            return;
        }
        if(h>500)  //高度
        {
            return;
        }
        document.getElementById("ads").style.display = "";
        document.getElementById("ads").style.height = h+"px";
        setTimeout("addCount()",30);
    }

    window.onload = function showAds()
    {
        addCount();
        setTimeout("noneAds()",7000); //停留时间自己适当调整
    }
    </script>

    <script type="text/javascript" language="javascript">
    var T = 500;
    var N = 500; //高度
    function noneAds()
    {
        if(T>0)
        {
            T--;
            N = N-5;
        }
        else
        {
            return;
        }
        if(N<0)
        {
            document.getElementById("ads").style.display = "none";
            return;
        }

        document.getElementById("ads").style.height = N+"px";
        setTimeout("noneAds()",30);
    }
    </script>
</head>
<body>
    <div id="ads" style="margin:auto; display:none; width:900px; top:0px; height:0px; border:solid 1px #000; background-color:#fafafa; overflow:hidden; text-align:center;">
        <a target="_blank" href="http://www.duzhi.net">网页教学网地址：http://www.duzhi.net</a>
    </div>
    <div style="margin:auto; width:900px; height:200px; border:solid 1px #000; background-color:#fafafa; text-align:center;">
        网站主体内容
    </div>
</body>
</html>