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RE: What's the problem? "Reuse of 1998 XHTML namespace is potentially misleading/wrong"

From: Larry Masinter <masinter@adobe.com>
Date: Mon, 16 Feb 2009 11:10:48 -0800
To: Maciej Stachowiak <mjs@apple.com>
CC: Lachlan Hunt <lachlan.hunt@lachy.id.au>, HTML WG <public-html@w3.org>
Message-ID: <8B62A039C620904E92F1233570534C9B0118C8658F04@nambx04.corp.adobe.com>
I'm glad we agree on the principles, but I don't think we agree on the remedy.
This relates to ISSUE-4 ("html-versioning") and ACTION-93:

It seems like we have four versioning mechanisms for content:

* MIME type
* namespaces
* version attribute on elements

All of these have been rejected as ways of distinguishing XHTML5 from XHTML2,
for various reasons which have been analyzed:


Only the namespace element is sufficient for distinguishing languages
in arbitrary generic XML  compound documents where there may not be even
the possibility to specify the HTML version.

So, I offer the following proposal:

HTML5 isn't being designed for incorporation in arbitrary generic XML compound
documents in any case.

So I would propose instead:
a)  XHTML5 *MUST NOT* be used in arbitrary generic XML compound documents. Any
   use of the namespace in such contexts denotes XHTML2, not XHTML5.
   Since HTML5 and XHTML5 are not being designed for inclusion in arbitrary
   XML compound documents, this should not have any impact in practice.

b) a new MIME type, application/xhtml5+xml, be allocated for XHTML5, and a new
   MIME type, application/xhtml2+xml, be allocated for XHTML2, since they both
   differ sufficiently from XHTML1.

   Since recommended practice is to deliver HTML5 as text/html in any case,
   this should not have any serious impact in practice either.


-----Original Message-----
From: Maciej Stachowiak [mailto:mjs@apple.com] 
Sent: Monday, February 16, 2009 10:43 AM
To: Larry Masinter
Cc: Lachlan Hunt; HTML WG
Subject: Re: What's the problem? "Reuse of 1998 XHTML namespace is potentially misleading/wrong"

On Feb 16, 2009, at 10:21 AM, Larry Masinter wrote:

> I think I wasn't clear enough, so let me try to clarify:
> In XML, namespaces identify vocabularies, not languages.
> Language definitions define rules for putting together
> vocabularies and user-supplied terms and procedures by
> which a receiver of an utterance in the language (a
> receiver) can interpret and understand the meaning
> intended by the sender of that utterance.
> You can have two different languages sharing the same
> namespace as long as there is sufficient information
> for any receiver of an utterance to determine which
> language was intended.
> Conclusion:
> XpMLq and XrMLs can share the same XML namespace
> as long as there is some way that you can determine
> which was intended, through MIME types, DOCTYPE
> or other means.
> This is true whether XpMLq and XrMLs are XHTML1
> and XHTML2, XHTML2 and XHML5 or XHTML5 and
> XHTML5.001 (an incremental update).
> Sharing namespaces with different languages is
> unacceptable if there is no other versioning
> mechanism than the namespace name. It would
> define two overlapping languages where there
> is no deterministic way of deciding which
> utterances were in which language, without
> extra contextual information. This is very
> bad language design.

XHTML2 and XHTML5 both use the application/xhtml+xml MIME type, and do  
not require a doctype declaration to be specified. XHTML2 has a  
version attribute on the root element, but it is optional.  
Furthermore, either may be used in a generic XML compound document  
where there may not be even the possibility to specify the HTML  
version via a root element attribute or doctype declaration. In  
practice user agents assign semantics and behavior almost exclusively  
based on the qualified name of an element, since there is no other  
disambiguation or versioning construct that is present in all contexts.

Thus, I believe the use of the same namespace by XHTML2 and XHTML5  
would be "unacceptable" and "very bad language design" by the standard  
you set out above.

> Whether or not there is any "vendor" who supports
> or is intending to support both XHTML2 and XHTML5
> is irrelevant to this question, since agents
> that cannot properly interpret an instance
> of the language they're presented with should
> not blindly try to interpret those instances
> in some other language.

I was trying to say politely that XHTML2 is unlikely to ever be widely  
implemented by mainstream Web software, so we shouldn't spend a lot of  
the group's time worrying about ways it may conflict with XHTML5. If  
someone were to invent a language that uses all the same words as  
English but with different meanings, then in theory that would be bad  
language design but that doesn't mean English speakers need to worry  
about it a whole lot.

To summarize, I definitely think there is a problem in theory, for the  
same reasons you gave above. But I don't think it will be a problem in  


> Larry
> --
> http://larry.masinter.net
> -----Original Message-----
> From: Maciej Stachowiak [mailto:mjs@apple.com]
> Sent: Thursday, February 12, 2009 4:19 PM
> To: Larry Masinter
> Cc: Lachlan Hunt; HTML WG
> Subject: Re: What's the problem? "Reuse of 1998 XHTML namespace is  
> potentially misleading/wrong"
> On Feb 11, 2009, at 12:46 PM, Larry Masinter wrote:
>>> But XHTML2 also has several major incompatibilities with XHTML1,
>>> which
>>> would effectively make it impossible to implement both XHTML 1.x
>>> and 2
>>> in the same implementation, if they share the same namespace [3].
>> The fact that one language makes <title>abc</title> equivalent
>> to <meta property="title" content="Document Title"/> and the other
>> does not does mean the languages are incompatible. But the elements
>> "title" and "meta" have the same meaning as vocabulary items, their
>> usage is different in the two languages.
>> I know that there is significant resistance to the idea that you  
>> might
>> define a vocabulary independent of a language which uses that
>> vocabulary,
>> or that you might define a language independent of the processing
>> rules
>> by which instances of text in the language should be processed, but
>> those separations are fundamental to the design of XML, and the
>> question
>> here is about XML namespaces and their use.
> What Lachlan raises is an actual technical problem in the design of
> the languages, even if the languages have "the same vocabulary" in
> some sense. It is not possible to implement a user agent that conforms
> to both XHTML5 and XHTML2. In many cases the same elements or
> attributes are defined to have different processing requirements.
> Since DTDs are optional, and since elements in the XHTML namespace may
> appear in a compound document, there is no way to determine which
> processing requirements to use. XHTML2 also has this problem with
> regard to XHTML1.x.
> I don't believe any vendor currently plans to support XHTML5 and
> XHTML2 together in the same product, so perhaps this is not a problem
> in practice. But it is still a design flaw in XHTML2 and should be
> fixed by the XHTML2 Working Group.
> Regards,
> Maciej
Received on Monday, 16 February 2009 19:11:32 UTC

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