Hey,

Code:
function start(){
	document.getElementById("timer").value++;
	timer = window.setTimeout("start()", 30);
}

function stop(){
	window.clearTimeout(timer);
	var resultaat = document.getElementById("timer").value;
	if(resultaat == 100){
		alert("Proficiat! Perfecte timing!");
		}
		else if((resultaat == 99) || (resultaat == 101)){
			alert("Bijna perfect, maar nog niet helemaal");
			}
			else{
				alert("Nog een beetje oefenen, je timing kan beter");
				}
}

HTML Code:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">

<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>Timing</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<link rel="stylesheet" type="text/css" href="timing.css" />
<script type="text/javascript">
/* <![CDATA[ */
function start(){
	document.getElementById("timer").value++;
	timer = window.setTimeout("start()", 30);
}

function stop(){
	window.clearTimeout(timer);
	var resultaat = document.getElementById("timer").value;
	if(resultaat == 100){
		alert("Proficiat! Perfecte timing!");
		}
		else if((resultaat == 99) || (resultaat == 101)){
			alert("Bijna perfect, maar nog niet helemaal");
			}
			else{
				alert("Nog een beetje oefenen, je timing kan beter");
				}
}
/* ]]> */
</script>
</head>
<body>
<div id="wrapper">
<form action="">
<fieldset>
	<legend>Timing</legend>
	<p>Probeer de timer te stoppen op <span>100</span></p>
	<input type="text" id="timer" value="0" onselect="blur()" />
	<div id="buttons">
		<input type="button" class="row" value="starten" onclick="start()" />
		<input type="button" class="row" value="stoppen" onclick="stop()" />
		<input type="reset" class="row" value="opnieuw" />
	</div>
</fieldset>
</form>
</div>
</body>
</html>
This gives an error in IE8, stating that on line 39 the object doesn't support this action.
HTML Code:
<input type="button" class="row" value="starten" onclick="start()" />
I don't see what is wrong with this, maybe you do?