Hai ragione avevo sbagliato la pagina!!!!...ma non funziona nemmeno cosi perchèèèè????!!!!!!

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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1">
<title>Inserisci utente</title>
<script language="JavaScript">
function controlla()
{
var ditta = document.theform.ditta.value;


if (ditta =="")
{
alert("il campo risulta vuoto");
return;
}
document.theform.submit();
}
</script>
</head>
<body>
<form action="do_insert.php" method="get" name="theform">
<table>
<tr>
<td align="right">Ditta:</td>
<td><input name="ditta" type="text" id="ditta" maxlength="80"></td>
</tr>

<tr>
<td collspan="2"><input type="button" name="button" value="inserisci" onClick="contolla();"></td>
</tr>
</table>
</form>
Home

</body>
</html>

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Proprio non riesco a capire.