在使用DHTML进行编程时比较有用，防止浏览器在请求中，死掉

var xml = GenXMLData(tableName, fieldID, "", "");
    var content = "<table height=100% width=100%><tr><td><div id='divDoing' style='position:absolute; left:500px; top:136px; width:196px; height:105px; z-index:1;'><div align='center'><p><img src='../Images/doing.gif' width='32' height='32'/></p><p><br/>正在查询、请稍候……</p></div></div></td></tr></table>";
    contentTD.innerHTML = content;
        var XmlHttp=new ActiveXObject("Microsoft.XMLHTTP");
    XmlHttp.onreadystatechange=function(){
        if(XmlHttp.readyState==4){
            if(XmlHttp.status==200)
            {
              contentTD.innerHTML = XmlHttp.responseText;
              firstPost = true;
            }
            else
            contentTD.innerHTML = content ;
        }
    }
    XmlHttp.open("post",webFile,true);
    XmlHttp.send(xml);