Con IE 6.0 fa lo submit.

codice:
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>
<head>
<title>Untitled</title>
</head>
<body>
<script language="JavaScript" type="text/javascript">
<!--
function isNum(unValore) {
 return isNaN(unValore) ? false : true;
}

function ctt(unTextField) {
 if (! isNum(unTextField.value)) {
  alert("Valore non numerico");
	return;
 }
 aForm = unTextField.parentElement;
 alert(aForm.tagName);
 aForm.submit();
}
//-->
</script>


<form id="pippo" action="pippo.php" method="post">
 <input type="text" id="t1">
 <input type="button" value="clicca" onclick="ctt(document.getElementById('t1'))">
</form>

</body>
</html>
L'alert che ho aggiunto mostra FORM