Dann bleibt nur warten.

<html>
<head>
<title>Testcounter</title>
<script type="text/javascript">
var aktiv = window.setInterval("countDown()",1000);
<!--
function countDown() {
t = parseInt(document.getElementById('counter').innerHTML);
document.getElementById('counter').innerHTML = t - 1;
if(parseInt(document.getElementById('counter').innerHTML) == 0){
window.clearInterval(aktiv);
document.getElementById('counter').innerHTML = 'Bum';
}
}
-->
</script>
</head>
<body>
<span id="counter">20</span>
</body>
</html>
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