JavaScript点击弹出浮动独立居中窗口

<script>
function open_box(){
document.all.ly.style.display="block";
document.all.ly.style.width=document.body.clientWidth;
document.all.ly.style.height=document.body.clientHeight;
}
function close_box(theForm){
document.all.ly.style.display='none';
return false;
}
</script>

<p align="center"><input type="button" value="弹出DIV" onClick="open_box()" /></p>
<!-- 浮层框架开始 -->
<div id="ly" align="center" style="position:absolute;z-index:3;display:none;">
<table width="540" height="300" border="1" cellpadding="0" cellspacing="0" style="border-collapse:collapse;" >
<tr>
<td style="background:#73A2d6;" valign="middle" height="10">
<div align="right"><a href="JavaScript:;" onclick="close_box();" style="color:#FFF;">关闭</a> </div></td>
</tr>
<tr>
<td height="130" align="center">这里是内容这里是内容这里是内容这里是内容这里是内容这里是内容这里是内容这里是内容</td>
</tr>
</table>
</div>
<!-- 浮层框架结束-->