改善用户体验的alert提示效果

页面都有弹出对话框以后，页面中其他部分颜色变暗的效果

<input type="button" value="点击这里" onclick="sAlert('测试效果<br>还可以直接书写HTML代码 <br>? 2006');" />

<script type="text/javascript" language="javascript">
function sAlert(str){
var msgw,msgh,bordercolor;
msgw=400;//提示窗口的宽度
msgh=100;//提示窗口的高度
bordercolor="#336699";//提示窗口的边框颜色
titlecolor="#99CCFF";//提示窗口的标题颜色

var sWidth,sHeight;
sWidth=document.body.offsetWidth;
sHeight=document.body.offsetHeight;

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";
document.body.appendChild(bgObj);
var msgObj=document.createElement("div")
msgObj.setAttribute("id","msgDiv");
msgObj.setAttribute("align","center");
msgObj.style.position="absolute";
msgObj.style.background="white";
msgObj.style.font="12px/1.6em Verdana, Geneva, Arial, Helvetica, sans-serif";
msgObj.style.border="1px solid " + bordercolor;
msgObj.style.width=msgw + "px";
msgObj.style.height=msgh + "px";
msgObj.style.top=(document.documentElement.scrollTop + (sHeight-msgh)/2) + "px";
msgObj.style.left=(sWidth-msgw)/2 + "px";
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>