<div style="position:absolute; top:0px; width:100px; height:100px; z-index:1; background-color: #009966;">置顶</div>
<div style="position:absolute; top:expression(document.body.clientHeight-this.offsetHeight); width:100px; height:100px; z-index:1; background-color: #33CC00;">沉底</div>
<div style="position:absolute; left:expression((document.body.clientWidth-this.offsetWidth)/2); top:expression((document.body.clientHeight-this.offsetHeight)/2); width:100px; height:100px; z-index:1; background-color: #996600">始终居中的层</div>
<div style="position:absolute; top:expression(document.body.clientHeight-this.offsetHeight); width:100px; height:100px; z-index:1; background-color: #33CC00;">沉底</div>
<div style="position:absolute; left:expression((document.body.clientWidth-this.offsetWidth)/2); top:expression((document.body.clientHeight-this.offsetHeight)/2); width:100px; height:100px; z-index:1; background-color: #996600">始终居中的层</div>
<head>
<title>Untitled Document</title>
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</head><body tracingsrc="T71397.jpg" tracingopacity="12">
<table><tr><td height="60" ></td></tr></table>
<table width="160" >
<tr>
<td align="center" width="50%" onMouseOver="MM_showHideLayers('hot1','','show')" onMouseOut="MM_showHideLayers('hot1','','hide')"> 热门栏目
<div id="Layer2" style="position:absolute; width:0; height:0; z-index:3; visibility: hidden;">
<div id="hot1" style="position:absolute; width:100px; height:80px; z-index:1; visibility: hidden; left: 3px; top: -68px;" >
<table width="100" >
<tr>
<td> 超级酷 </td>
</tr>
<tr>
<td> 软硬兼师 </td>
</tr>
<tr>
<td> 蓝天BBS </td>
</tr><tr>
<td> 网络纵横 </td>
</tr>
</table>
</div>
</div>
</td>
</tr>
<tr>
<td align="center" width="50%" onMouseOver="MM_showHideLayers('hot2','','show')" onMouseOut="MM_showHideLayers('hot2','','hide')"> 电脑网络
<div id="Layer1" style="position:absolute; z-index:2; visibility: hidden; width: 0; height: 0;">
<div id="hot2" style="position:absolute; width:100px; z-index:1; visibility: hidden;" class="pos">
<table >
<tr>
<td> 科技超市 </td>
</tr>
<tr>
<td> 网探星期 </td>
</tr> <tr>
<td> 城市话题 </td>
</tr>
<tr>
<td><a href="layer.asp"> 城市话题 </a></td>
</tr>
<tr>
<td> 科技超市 </td>
</tr>
<tr>
<td> 网探星期 </td>
</tr> <tr>
<td> 城市话题 </td>
</tr>
<tr>
<td><a href="layer.asp"> 城市话题 </a></td>
</tr>
</table>
</div>
</div>
</td>
</tr>
</table>
</body>
</html>
现在我想使由单元格所引出的层的底部（层里的表格的最下面一列）始终与该单元格对齐，这应该怎么做？