请教：怎样获取网页中某一单元格的坐标

<div align="center">
<table border="0" cellpadding="0" style="border-collapse: collapse" width="98%" id="table1">
<tr>
<td>　</td>
<td>　</td>
<td>　</td>
</tr>
<tr>
<td id="td1">　</td>
<td id="td2">　</td>
<td id="td3">　</td>
</tr>
<tr>
<td>　</td>
<td>　</td>
<td>　</td>
</tr>
</table>
</div>
<script language="javascript">
<!--
function showXY(o)
{
alert(o.getBoundingClientRect().top + "\r\n" +o.getBoundingClientRect().left );
}
showXY(td1);
showXY(td2);
showXY(td3);

//-->
</script>

<div align="center">
<table border="0" cellpadding="0" style="border-collapse: collapse" width="98%" id="table1">
<tr>
<td onMouseOver="javascript:Show(this);">AAA</td>
<td onMouseOver="javascript:Show(this);">BBB</td>
<td onMouseOver="javascript:Show(this);">CCC</td>
</tr>
</table>
</div>
<script language="javascript">
<!--
function showXY(obj)
{
alert(obj.getBoundingClientRect().top + "\r\n" +obj.getBoundingClientRect().left );
}
</script>