JS弹出(DIV)窗口,背景为半透明

<script type="text/javascript" language="javascript">
function sAlert(){
var str="test弹窗效果trybb.com";
var msgw,msgh,bordercolor;  
msgw=400;//提示窗口的宽度  
msgh=100;//提示窗口的高度  
titleheight=25 //提示窗口标题高度  
bordercolor="#c51100";//提示窗口的边框颜色  
titlecolor="#c51100";//提示窗口的标题颜色  

var sWidth,sHeight;  
sWidth=screen.width;  
sHeight=screen.height;  

var bgObj=document.createElement("div");  
bgObj.setAttribute('id','bgDiv');  
bgObj.style.position="absolute";  
bgObj.style.top="0";  
bgObj.style.background="#cccccc";  
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="white";  
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").appendChild(title);  
var txt=document.createElement("p");  
txt.style.margin="1em 0"
txt.setAttribute("id","msgTxt");  
txt.innerHTML=str;  
document.getElementById("msgDiv").appendChild(txt);  
}  
</script>

HTML代码:
<input type="button" value="点击这里" onClick="sAlert();" />