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Re: [RDFa] ISSUE-3 @class and @role for rdf:type

From: Mark Birbeck <mark.birbeck@x-port.net>
Date: Mon, 9 Jul 2007 16:10:51 +0100
Message-ID: <640dd5060707090810i4365dd4dmb71252bfa60adde9@mail.gmail.com>
To: "Niklas Lindström" <lindstream@gmail.com>
Cc: "Ben Adida" <ben@adida.net>, public-rdf-in-xhtml-tf@w3.org

Hi Niklas,

Very useful comments, many thanks. I hope you don't mind if I just
reply to a couple of points that particularly caught my eye, in
particular the following:

> [snip]
> But the issue for me is that the usage of @role, as far as I can
> interpret it, is to define a relation to a resource *with the subject
> being the element itself*. If this is the case, @role is out.


> [snip]
> One practical reason is that @class seems today to be about attaching
> information about the element itself (akin to how I interpret @role
> above). It may not be entirely clear, but considering a designer
> aiming to state some stuff about a div containing a name and phone
> number, '<div class="person">' is perhaps best interpreted as "this
> element is a person data container".

This is a very interesting point. Along very similar lines, Ben
mentioned a while back that one of the things that TBL didn't like
about the use of @class in RDFa was that he felt that when he uses
@class in his documents, he is actually saying something about the
document element itself, and not the 'Person' or 'Event' being alluded
to. Your comments seem to me to be referring to exactly this issue.

The compromise we made with TBL's comments was to say that only
prefixed values in @class should be parsed by an RDFa parser, but in a
way that is worse than parsing all values. This is because--if we use
the ideas you're referring to here--we now have one range of @class
values that are in reference to elements in the document, and another
range of @class values that apply to meta-information. In other words
we've made the attribute have two different meanings based on its
value, which is not good.

At the risk of seeming to be dealing with angels jumping up and down
on pin-heads, I'll venture to put this issue into its RDF context.

If we think first of RDF/XML, it is not actually possible in RDF/XML
to say anything about the document that carries the RDF/XML. So if you
have (to continue with an earlier example...sorry Ivan!):


any statement you ever make using that URI will always be about the
resource Ivan, and never about the XML document itself.

Now, you could say that RDFa has gone somewhat in that direction,
although not quite as far.

As you rightly allude to Niklas, the way we have things at the moment
in RDFa is that, whilst statements are 'carried' by an HTML/XHTML
document, they are not generally _about_ the document itself--they are
usually about 'People' and 'Events' which are being referred to by the

However, unlike an RDF/XML document, with RDFa it *is* possible to
make statements about the document itself. RDFa gives us a chimera; it
can be both a web document (an information resource in RDF parlance)
and at the same time it can carry metadata about resources (equivalent
to the RDF/XML side).

You rightly draw attention to @role; that's a very good illustration
of how we fully intended to maintain this distinction. @role was
always meant to be about the ability to say things like "the purpose
of this script element is to provide a hint to a user"; as you say,
that's saying something about an element in a document, and is very
different from saying things like "some external document is a

So although I think we do need an RDFa 'view' on what @class and @role
do, I think we're not really in a position to say quite what it should
be at the moment; should these attributes only provide metadata about
the information that our HTML document is 'carrying' or should they
provide information about the document itself, in the way that @role
was originally intended? We've been quite careful in other places to
watch what we do with @id, for example, and I think your comments are
a useful reminder that we should continue to take care that we don't
close for the future the ability to say useful things about the
document itself.

> 3. Regarding a new attribute. Not remembering all suggestions, I
> suggest "@instanceof" - not the least since the rdfs:comment of
> rdf:type states "The subject is an instance of a class". Meshed
> example:
>     <div about="#jane" class="person" instanceof="foaf:Person">
>         <span class="fn" property="vcard:fn">Jane Doe</span>
>     </div>

That's about the best suggestion so far, I think. :)



  Mark Birbeck, formsPlayer

  mark.birbeck@x-port.net | +44 (0) 20 7689 9232
  http://www.formsPlayer.com | http://internet-apps.blogspot.com

  standards. innovation.
Received on Monday, 9 July 2007 15:10:58 UTC

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