JavaScript修改alert样式的方法介绍

<script language="javascript" type="text/javascript">
    window.alert = function(txt)
    {
     var shield = document.createElement("DIV");
     shield.id = "shield";
     shield.style.position = "absolute";
     shield.style.left = "0px";
     shield.style.top = "0px";
     shield.style.width = "100%";
     shield.style.height = document.body.scrollHeight+"px";
     shield.style.background = "#333";
     shield.style.textAlign = "center";
     shield.style.zIndex = "10000";
     shield.style.filter = "alpha(opacity=0)";
     var alertFram = document.createElement("DIV");
     alertFram.id="alertFram";
     alertFram.style.position = "absolute";
     alertFram.style.left = "50%";
     alertFram.style.top = "50%";
     alertFram.style.marginLeft = "-225px";
     alertFram.style.marginTop = "-75px";
     alertFram.style.width = "450px";
     alertFram.style.height = "150px";
     alertFram.style.background = "#ccc";
     alertFram.style.textAlign = "center";
     alertFram.style.lineHeight = "150px";
     alertFram.style.zIndex = "10001";
     strHtml = "<ul style="list-style:none;margin:0px;padding:0px;width:100%"> ";
     strHtml += " <li style="background:#DD828D;text-align:left;padding-left:20px;font-size:14px;font-weight:bold;height:25px;line-height:25px;border:1px solid #F9CADE;">[系统提示]</li> ";
     strHtml += " <li style="background:#fff;text-align:center;font-size:12px;height:120px;line-height:120px;border-left:1px solid #F9CADE;border-right:1px solid #F9CADE;">"+txt+"</li> ";
     strHtml += " <li style="background:#FDEEF4;text-align:center;font-weight:bold;height:25px;line-height:25px; border:1px solid #F9CADE;">
     <input type="button" value="确 定" onclick="doOk()" /></li> ";
     strHtml += "</ul> ";
     alertFram.innerHTML = strHtml;
     document.body.appendChild(alertFram);
     document.body.appendChild(shield);
     var c = 0;
     this.doAlpha = function(){
         if (c++ > 20){clearInterval(ad);return 0;}
         shield.style.filter = "alpha(opacity="+c+");";
     }
     var ad = setInterval("doAlpha()",5);
     this.doOk = function(){
         alertFram.style.display = "none";
         shield.style.display = "none";
     }
     alertFram.focus();
     document.body.onselectstart = function(){return false;};
    }
</script>