定位，和js的他处的层

<div id="dd" style="background-color:white; border:2px solid #666; width:300px;height:100px;position:absolute;right:600px;top:400px;z-index:101;display:none">
       <div style="background-image:url(images/transparent.gif);margin-top:40px;margin-left:10px;magin-right:10px;">         
         <input type="button" value="保 留" onclick="reserve('1');" style="cursor:hand" onmouseover="this.style.color='#FFFFFF'" onmouseout="this.style.color='#000000'" class="btn_login_grid"/>
         <input type="button"  value="不保留" onclick="reserve('0');" style="cursor:hand" onmouseover="this.style.color='#FFFFFF'" onmouseout="this.style.color='#000000'" class="btn_login_grid"/>
         <input type="button" id="btncancel" value="取 消" style="cursor:hand" onmouseover="this.style.color='#FFFFFF'" onmouseout="this.style.color='#000000'" class="btn_login_grid"/>
        </div>
 </div>
 <div id="ifrdiv" style="filter:alpha(opacity=50);opacity:0.5;background-color:#000000;display:none">   
<iframe width="100%" height="800" id="frame" style="filter:alpha(opacity=50);opacity:0.5;background-color:#000000" ></iframe>
<div>

 

js--------------
 var bW=document.body.scrollWidth;
 var bH = document.body.scrollTop+document.body.offsetHeight+100;
 $("#ifrdiv").css({width:bW,height:bH,top:"0px",left:"0px",position:"absolute"});
 $("#ifrdiv").css("z-index",100);
 $("#ifrdiv").show();
 $("#frame").css({width:bW,height:bH}); 
 $("#dd").show();

----用jquery写的。