a question about WWW-Authenticate header

From: Manlio Perillo <manlio.perillo@gmail.com> Date: Sun, 21 Dec 2008 15:25:29 +0100 Message-ID: <494E51D9.8040209@gmail.com> To: www-talk@w3.org · This archive was generated by hypermail 2.2.0+W3C-0.50 : Tuesday, 27 October 2009 08:38:52 GMT

Hi.

What is the "correct" behaviour for an HTTP user agent, when it process 
a WWW-Authenticate header containing unsupported challenge?

I have found that Internet Explorer, Firefox, Konqueror, Epiphany, 
ignore the header.

Opera, however, reports an error:

Error!
The server requested a login authentication method that is not supported.

Is this an acceptable behaviour?
RFC 2616 says nothing, here.

Thanks   Manlio Perillo