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>