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Re: RDFa in HTML 5

From: Shelley Powers <shelleyp@burningbird.net>
Date: Fri, 22 May 2009 09:15:49 -0500
Message-ID: <4A16B395.4060003@burningbird.net>
To: Philip Taylor <pjt47@cam.ac.uk>
CC: RDFa mailing list <public-rdf-in-xhtml-tf@w3.org>, HTML WG <public-html@w3.org>
Actually I didn't fully quote one of Philip's emails, so am replicating 
his email here (again with his permission):

Shelley Powers wrote:
> [...]
> PS I will say one thing, and I'm parroting Henri in this regard, to me 
> a conforming implementation of RDFa in HTML5 is not necessarily one 
> that only meets what's required for HTML5 -- it has to meet a 
> conformance requirement for RDF, too.How would we know if the document 
> is conforming? Because the same annotation in a document served up as 
> XHTML5, should generate the exact same RDF graph, as would be 
> generated if the document is served up as HTML5. To ensure this, how 
> the annotation is interpreted from a data perspective must be defined 
> in a single document, such as RDFa-in-XHTML.

That's something I'd definitely agree with. E.g. when someone implements 
an RDFa parser with JS in a web browser, they should use the same code 
to extract data regardless of whether the document was originally 
text/html or application/xhtml+xml, and get the same output. That means 
there needs to be just one specification saying how to extract data, to 
avoid the conflicts you mention.

> [...]
> The reason why Shane's document is "sparse" on parsing  (processing) 
> information (according to the WhatWG IRC entries) is that Shane was 
> deferring the RDFa processor conformance to the RDFa-XHTML syntax and 
> processing document. This was right and proper. He was using good 
> technique.

The problem in that document is it doesn't define how to map from the 
syntax onto the RDFa-in-XHTML processing model, which leaves a gap where 
the behaviour is undefined. E.g. I can write <div xmlns:="..."> in HTML, 
and I don't know whether that attribute should be ignored or should 
redefine the default prefix mapping, because it's impossible in XHTML 
and so the RDFa-in-XHTML specification doesn't explain how to handle it.

One idea for fixing the gap is to produce a more detailed mapping from 
text/html onto the RDFa-in-XHTML processing model. But that seems like 
an unpleasantly difficult solution, since RDFa-in-XHTML wasn't really 
designed to be used like that and there lots of small mismatches and 
edge cases that make it tricky.

Since HTML 5 already defines how to handle text/html and 
application/xhtml+xml in a common processing model, I think redefining 
the RDFa processing model on top of the HTML 5 processing model is 
possibly the best way to get well-defined, consistent behaviour between 
HTML and XHTML. So it would entirely replace the current RDFa-in-XHTML 
spec, ensuring there's only a single document telling people how to 
parse RDFa in both HTML and XHTML. Maybe it should be thought of as a 
new edition of the existing spec, rather than a totally new spec.

I guess there are lots of political/process issues with doing that, but 
it'd be nice to have a technically sound solution before getting blocked 
by those issues.

-- 
Philip Taylor
Received on Friday, 22 May 2009 14:16:32 GMT

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