Yang harus ada, object "loading", JS Ajax ini udah di tes di opera 9 dan sukses..
//------------------------------------------------------------------

function showHint(a,b)
{
if (a.length > 0)
{
var url="ajax.function.php?" + a
targetnya = b
xmlHttp=GetXmlHttpObject(stateChanged)
xmlHttp.open("GET", url , true)
xmlHttp.send(null)
}
else
{
targetnya = b
document.getElementById(b).innerHTML=""
}
}

function clearTarget(c)
{
document.getElementById(c).innerHTML=""
}

function stateChanged()
{
if (xmlHttp.readyState==4 || xmlHttp.readyState=="complete")
{
document.getElementById(targetnya).innerHTML=xmlHttp.responseText
document.getElementById('loading').style.display = "none";
} else {
document.getElementById('loading').style.display = "block";
}
}

function GetXmlHttpObject(handler) {
{
var xmlHttp;
try
{
// Firefox, Opera 8.0+, Safari
xmlHttp=new XMLHttpRequest();
}
catch (e)
{
// Internet Explorer
try
{
xmlHttp=new ActiveXObject("Msxml2.XMLHTTP");
}
catch (e)
{
try
{
xmlHttp=new ActiveXObject("Microsoft.XMLHTTP");
}
catch (e)
{
alert("Your browser does not support AJAX!");
return false;
}
}
}
xmlHttp.onreadystatechange=handler
return xmlHttp
}
}