W3C home > Mailing lists > Public > public-rdf-in-xhtml-tf@w3.org > September 2009

Re: PROPOSAL: Errata text to deal with the issue of predeclared 'xml' and 'xmlns' prefixes

From: Philip Taylor <pjt47@cam.ac.uk>
Date: Fri, 25 Sep 2009 15:09:30 +0100
Message-ID: <4ABCCF1A.7010004@cam.ac.uk>
To: Julian Reschke <julian.reschke@gmx.de>
CC: Shane McCarron <shane@aptest.com>, "public-rdf-in-xhtml-tf.w3.org" <public-rdf-in-xhtml-tf@w3.org>
Julian Reschke wrote:
> Clarifying... this is *really* academic, right? Or are you expecting 
> people to actually use things which are in the "xml" or "xmlns" namespaces?

It's not implausible that they *could* use them, e.g. you might use a 
RDDL extractor on http://www.w3.org/XML/1998/namespace and get a triple 

     <http://www.w3.org/TR/REC#sec-lang-tag> .

and then serialise it to RDFa (either manually, or with a tool that 
automatically CURIEfies URIs) using <p about="[xml:]" ...> if you think 
that prefix is predeclared.

(Fun fact: applying http://www.rddl.org/rddl2rdf.xsl to 
http://www.w3.org/XML/1998/namespace actually results in 
non-namespace-well-formed output, because it generates xmlns="$baseURI". 
XML Namespaces are hard.)

But I agree it's very rare and unlikely to affect real users in a 
noticeable way, so it's an academic point.

But I think it's an important academic point, for two reasons:

Firstly, given 
RDFa's specified behaviour here will determine whether it's possible or 
impossible to implement a conforming RDFa processor in XSLT, or in other 
technologies with similar constraints. (This is assuming you have a 
conforming XSLT processor - according to 
it's already impossible to implement RDFa at all in one widely-deployed 
XSLT implementation.)

Secondly, it affects the conceptual relationship between RDFa and XML 
Namespaces. For example, 

> [Prefix] mappings could have been provided using the attribute @banana,
> containing syntax like "ex=http://example.org".
> (And there is discussion about providing some additional mechanism to
> provide these mappings.)
> But for now, the only mechanism available is that any attribute that
> conforms to the pattern described in [XMLNS], is interpreted as
> providing a prefix mapping.

similarly says:

> I hope that we were very careful in the Recommendation to indicate that
> it is the *syntax* of the XML Namespace declarations that is used to
> define RDFa prefix mappings. [...] We don't use XML Namespaces.

If, in fact, the RDFa prefix mappings depend on XML Namespaces' own 
default bindings for xml/xmlns prefixes, then it is untrue that RDFa 
merely uses the Namespaces attribute syntax, and so this represents a 
shift in the view of the relationship between RDFa and Namespaces. So I 
think it's important to be clear about this and to understand the 

Also, it's an opportunity to demonstrate that XML Namespaces cause pain, 
which is always fun.

Philip Taylor
Received on Friday, 25 September 2009 14:10:07 UTC

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