Do sekce <BODY> umístěte:
<script language="JavaScript">
var sekund = 20;

function leave() {
if (0 == sekund)
alert('Konec')
else {

sekund -= 1;
document.all.countdown.innerText = sekund + " ";
setTimeout("leave()", 1000);
}
}
</script>


<body onload="setTimeout('leave()', 1000)">
<span ID="countdown"><script language="JavaScript">
document.write(sekund);
</script>
</span>