Puoi fare cosi....
codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>Untitled</title>
<style type="text/css">
<!--
.hidden {position:absolute;top:-1000;left:-1000;width:400;height:400;background-color:yellow}
.show {position:absolute; top:100; left: 100;width:400;height:400; background-color:yellow}
-->
</style>
<script language="JavaScript" type="text/javascript">
<!--
function wait(nSeconds) {
show();
setTimeout("hide()",nSeconds*1000);
} // function wait(nSeconds)
function show() {
document.getElementById('banner').className = "show";
} // function show()
function hide() {
document.getElementById('banner').className = "hidden";
} // function hide()
//-->
</script>
</head>
<body onload="wait(10)">
<div id="banner" class="hidden">
Testo della pubblicità che scomparirà fra 10 secondi
</div>
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</body>
</html>