关于js问题请求大家帮忙

程序代码：

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www. xmlns="http://www. http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>显示、隐藏弹出层的例子</title>
<style type="text/css">
body{
    color:#008000;
    font:12px/1.5 arial;
}
a{
    color:#0B59B2;
    margin:0 8px;
}
#box{
    position:absolute;
    width:230px;
    height:60px;
    background:#F5F9FA;
    border:2px solid #95B5D5;
    display:none;
}
</style>
<script type="text/javascript">
window.onload = function ()
{
    var aA = document.getElementsByTagName("a");
    var oBox = document.getElementById("box");
    var timer = null;
    var i = 0;
    for (; i < aA.length; i++)
    {
        aA[i].onmouseover = function ()
        {
            clearTimeout(timer);
            var oParent = this.parentElement || this.parentNode;
            oBox.style.top = pageY(this) + this.offsetHeight + 5 + "px";
            oBox.style.left = pageX(oParent) + "px";
            oBox.style.display = "block"
        };
        aA[i].onmouseout = oBox.onmouseout = function ()
        {
            timer = setTimeout(function ()
            {
                oBox.style.display = "none"
            }, 300)
        };
        oBox.onmouseover = function ()
        {
            clearTimeout(timer);
        }
    }
};
function pageX (elem)
{
    return elem.offsetLeft + (elem.offsetParent ? arguments.callee(elem.offsetParent) : 0)
}
function pageY (elem)
{
    return elem.offsetTop + (elem.offsetParent ? arguments.callee(elem.offsetParent) : 0)
}
</script>
</head>
<body>
<p>作者：<a href="javascript:;">swiss原子弹</a>发表日期：211-8-17 21:56:00</p>
<p>作者：<a href="javascript:;">可乐马驹</a>发表日期：211-8-17 21:56:00</p>
<p>作者：<a href="javascript:;">网友网友love</a>发表日期：211-8-17 21:56:00</p>
<div id="box"></div>
</body>
</html>

其中

var aA = document.getElementsByTagName("a");

指定的是a标记我想这样

<p>作者：<a href="javascript:;">swiss原子弹</a>发表日期：211-8-17 21:56:00</p>

给a标记定义一个id

<p>作者：<a id="pop" href="javascript:;">swiss原子弹</a>发表日期：211-8-17 21:56:00</p>

这样怎么做才能是这段程序运行成功

关于js问题 请求大家帮忙