codice:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
<head>
    <title>Pagina senza titolo</title>

    <script language="javascript" type="text/javascript">
// <!CDATA[

window.onload = function()
{
    document.forms[0].onsubmit = function()
    {
        alert("questa funziona sovrascrive l'altra");
    }

}


// ]]>
    </script>
</head>
<body>
<form onsubmit="alert('mia funzione tutta sbagliata o inesistente');">
    <input id="Submit1" type="submit" value="submit" />
</form>
</body>
</html>