当页面弹出一个层时,层后面的内容变灰,突出显示当前层效果
当页面弹出一个层时,层后面的内容变灰,突出显示当前层效果,就像WEB迅雷的设置界面或126邮箱那样的效果是如何做的.

------解决方案--------------------
手头上有个，凑合用吧。
<script type= "text/javascript " language= "javascript ">
sAlert( ' <table> <tr> <td> 内容 </td> </tr> </table> ');//调用窗口

function sAlert(str){
var msgw,msgh,bordercolor;
msgw=400;//提示窗口的宽度
msgh=200;//提示窗口的高度
titleheight=25 //提示窗口标题高度
bordercolor= "#dddddd ";//提示窗口的边框颜色
titlecolor= "#999999 ";//提示窗口的标题颜色

var sWidth,sHeight;
sWidth=document.body.offsetWidth;
sHeight=screen.height;
var bgObj=document.createElement( "div ");
bgObj.setAttribute( 'id ', 'bgDiv ');
bgObj.style.position= "absolute ";
bgObj.style.top= "0 ";
bgObj.style.background= "#777 ";
bgObj.style.filter= "progid:DXImageTransform.Microsoft.Alpha(style=3,opacity=25,finishOpacity=75 ";
bgObj.style.opacity= "0.6 ";
bgObj.style.left= "0 ";
bgObj.style.width=sWidth + "px ";
bgObj.style.height=sHeight + "px ";
bgObj.style.zIndex = "10000 ";
document.body.appendChild(bgObj);

var msgObj=document.createElement( "div ")
msgObj.setAttribute( "id ", "msgDiv ");
msgObj.setAttribute( "align ", "center ");
msgObj.style.background= "white ";
msgObj.style.border= "1px solid " + bordercolor;
msgObj.style.position = "absolute ";
msgObj.style.left = "50% ";
msgObj.style.top = "50% ";
msgObj.style.font= "12px/1.6em Verdana, Geneva, Arial, Helvetica, sans-serif ";
msgObj.style.marginLeft = "-225px " ;
msgObj.style.marginTop = -75+document.documentElement.scrollTop+ "px ";
msgObj.style.width = msgw + "px ";
msgObj.style.height =msgh + "px ";
msgObj.style.textAlign = "center ";
msgObj.style.lineHeight = "25px ";
msgObj.style.zIndex = "10001 ";

var title=document.createElement( "h4 ");
title.setAttribute( "id ", "msgTitle ");
title.setAttribute( "align ", "right ");
title.style.margin= "0 ";
title.style.padding= "3px ";
title.style.background=bordercolor;
title.style.filter= "progid:DXImageTransform.Microsoft.Alpha(startX=20, startY=20, finishX=100, finishY=100,style=1,opacity=75,finishOpacity=100); ";
title.style.opacity= "0.75 ";
title.style.border= "1px solid " + bordercolor;
title.style.height= "18px ";
title.style.font= "12px Verdana, Geneva, Arial, Helvetica, sans-serif ";
title.style.color= "black ";
title.style.cursor= "pointer ";
title.innerHTML= "关闭 ";
title.onclick=function(){
document.body.removeChild(bgObj);
document.getElementById( "msgDiv ").removeChild(title);
document.body.removeChild(msgObj);
}
document.body.appendChild(msgObj);
document.getElementById( "msgDiv ").appendC