Tack för svaret, testade följande kod, funkar bra som den är här, men inte i .ascx sidan...
<?xml version="1.0" encoding="UTF-8"?>
<!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" xml:lang="en" lang="en">
<head>
<title> </title>
<script type="text/javascript">
</script>
<link href="scriptstyle.css" rel="stylesheet" type="text/css" />
</head>
<body>
<div id="textqty">Antal:</div>
<form name="f1" class='qtybg'>
<input name='qty' type='text' class='qtyfield' id='qty' size="1" />
<ul id="navigation">
[*]<input type='button' name='add' class='addbtn' onclick='javascript
: document.getElementById("qty").value++;' value=''/>
[*]<input type='button' name='subtract' class='subtractbtn' onclick='javascript
: document.getElementById("qty").value--;' value=''/>[/list] </form>
</body>
</html>