Media types and namespaces (was Re: a fly-by of Compound Document (WICD) specs w.r.t. TAG issues)

On Tuesday 2005-11-22 01:13 -0600, Dan Connolly wrote:
> Re
>  nsMediaType-3: Relationship between media types and namespaces?
>  http://www.w3.org/2001/tag/issues.html?type=1#nsMediaType-3
>  and nearby mediaTypeManagement-45
> 
> they seem to take a conservative position; they're not mixing
> namespaces in this version:

True for this version, but not for (current plans for) the next.

The current processing model used by the implementors in the CDF group
for the interaction of media-type dispatch and namespace dispatch is
that XML media types all dispatch to a system that does namespace
dispatch, with some media types implying additional constraints on that
system.  This part seems relatively noncontroversial, at least within
CDF (see, e.g., discussion in [1]).

The bigger controversy seems to be not over the processing model but
over identification of (and content-negotiation for) documents that mix
namespaces.  I think there are a bunch of questions the CDF group needs
to answer for this (which I'm restating from [2]).  The first two are
largely research questions over existing specifications ("foo"):

 1. Does the application/foo+xml registration require that
    all elements in a document with that type be FooML?  If it
    does, how must applications handle the case where some are not?

 2. Does the application/foo+xml registration require that
    the root element in a document with that type be FooML?  If
    it does, how must applications handle the case where it is not?

Trying to answer these for existing specifications is not necessarily
trivial.  For example, XHTML1 does not require conforming user agents to
conform to Namespaces in XML [3], and the media type registration only
vaguely refers to the user agent conformance anyway [4].  The SVG media
type registration [5] answers "no" to the first question but is unclear
on the second question (although I'm not happy about that [6]).

Answering these (and perhaps developing suggestions on how media type
registrations should answer them) may help answer the harder questions
that I think the CDF group is going to have to address in the next phase
of its work:

 3. What MIME type(s) should be used for documents that mix namespaces
    from multiple document format specifications?

 4. Should it be possible for content negotiation regarding which
    document formats may be combined in a single document to occur
    independently of profiles?  (I think it should.)  If so, how is this
    done?

 5. What does indicating acceptance of a document format's media type
    mean in terms of accepting it within compound documents?  It clearly
    implies acceptance as the only document format.  What about as the
    root format within a multi-format document?  What about as a
    non-root format within a multi-format document?

-David

[1] http://www.w3.org/2005/10/28-cdf (search for "dispatch")
[2] http://lists.w3.org/Archives/Member/member-cdf/2005Oct/att-0181/cdi-req
[3] http://www.w3.org/TR/2002/REC-xhtml1-20020801/#uaconf
[4] http://www.ietf.org/rfc/rfc3236.txt
[5] http://www.w3.org/TR/2005/WD-SVGMobile12-20050413/mimereg.html
[6] http://lists.w3.org/Archives/Public/www-svg/2004Nov/0046

-- 
L. David Baron                                <URL: http://dbaron.org/ >
           Technical Lead, Layout & CSS, Mozilla Corporation