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FW: Short briefing/background doc't regarding RDFa, prefixes and HTML

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  • misha.wolf@thomsonreuters.com
    fyi ... From: Wolf, Misha (M Cont Ent) Sent: 03 February 2011 09:50 To: Henry S. Thompson ; www-tag@w3.org Subject: RE: Short briefing/background doc t
    Message 1 of 3 , Feb 3, 2011
      fyi

      -----Original Message-----
      From: Wolf, Misha (M Cont Ent)
      Sent: 03 February 2011 09:50
      To: 'Henry S. Thompson'; www-tag@...
      Subject: RE: Short briefing/background doc't regarding RDFa, prefixes
      and HTML

      A reminder that the IPTC and its member News Agencies are using very
      widely the IPTC's alternative to CURIEs and QNames, called QCodes.
      The differences between CURIEs and QCodes are:

      - The QCode spec requires that the URI corresponding to the alias
      is a valid URI. The CURIE spec, IIRC, does not.

      - The QCode spec explicitly allows the right-hand side not to be a
      legal XML Name. I don't know what the CURIE spec says about this.

      - While the creators of RDFa stated that the current mechanism for
      alias definition is to be considered simply one of a set of
      possible mechanisms, I think that it is currently the only
      specified mechanism. We (the IPTC) rejected that mechanism as
      too verbose and created, instead, our catalog.

      Misha Wolf
      Standards Manager, Metadata Group, Thomson Reuters

      -----Original Message-----
      From: www-tag-request@... [mailto:www-tag-request@...] On Behalf
      Of Henry S. Thompson
      Sent: 03 February 2011 09:35
      To: www-tag@...
      Subject: Short briefing/background doc't regarding RDFa, prefixes and
      HTML

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      I've prepared a short introduction [1] to this issue in preparation
      for possible discussion at the upcoming TAG f2f. Comments welcome.

      ht

      [1] http://www.w3.org/2001/tag/doc/RDFa_HTML_prefix_issue.html
      - --
      Henry S. Thompson, School of Informatics, University of Edinburgh
      10 Crichton Street, Edinburgh EH8 9AB, SCOTLAND -- (44) 131
      650-4440
      Fax: (44) 131 651-1426, e-mail: ht@...
      URL: http://www.ltg.ed.ac.uk/~ht/
      [mail from me _always_ has a .sig like this -- mail without it is
      forged spam]
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    • misha.wolf@thomsonreuters.com
      fyi ... From: www-tag-request@w3.org [mailto:www-tag-request@w3.org] On Behalf Of Nathan Sent: 03 February 2011 12:03 To: Henry S. Thompson Cc: www-tag@w3.org
      Message 2 of 3 , Feb 3, 2011
        fyi

        -----Original Message-----
        From: www-tag-request@... [mailto:www-tag-request@...] On Behalf
        Of Nathan
        Sent: 03 February 2011 12:03
        To: Henry S. Thompson
        Cc: www-tag@...
        Subject: Re: Short briefing/background doc't regarding RDFa, prefixes
        and HTML

        Hi Henry,

        /Personal/ comments indented, note that I'm not speaking on behalf of
        the RDFa WG here.

        Henry S. Thompson wrote:
        > I've prepared a short introduction [1] to this issue in preparation
        > for possible discussion at the upcoming TAG f2f. Comments welcome.
        >
        > [1] http://www.w3.org/2001/tag/doc/RDFa_HTML_prefix_issue.html


        RDFa uses CURIEs extensively as the values of attributes. The
        non-empty prefixes in those CURIEs are interpreted relative to

        You've missed @profile which allows the importing of prefix and
        term definitions from an external profile (read: hosted on the web
        somewhere), also provides a default RDFa processor profile (meaning
        some prefixes are always supported and mapped to a specific uri),
        and also means that if a specified profile can't be dereferenced and
        read correctly (yes at parse / process time) then the element on
        which the @profile is declared, and all child elements, are skipped
        (no triples will be extracted from these elements).


        Neither proposal contains any concrete evidence about the utility, or
        lack of it, of prefixes for authors, or the importance, or lack of it,
        of that utility to authors and consequently to uptake.

        Very roughly, RDFa is at the convergence point between XML/XHTML,
        RDF and HTML, in both the RDF and HTML worlds authors find it very
        beneficial to be able to write simple string tokens like "foaf:name"
        or "title" rather than using full URIs (which people often get wrong
        and which increases the bandwidth required to author, send and
        receive RDF(a)).

        HTML typically uses well defined simple tokens in @rel like "author"
        and "stylesheet" which have universal meaning (in html and Web
        Linking at least), and both authors and consumers treat these as
        simple tokens not paired to URIs.

        RDF uses web names (URIs) for properties / relations, so that they
        can be looked up (via dereferencing) by humans / machines to get
        a description of the property. This allows new vocabularies and
        properties to be created at any time, and for the range of things
        that can be described with RDF to be unbounded in a webized and
        extensible manner.

        RDF authors practically need to be able to write "foaf:name" in
        serializations and have it resolve to a full URI when being
        processed.

        Due to RDF's XML heritage qnames and the XML prefix approach was
        adopted, likewise RDFa+XHTML which has heritage in both RDF and
        XHTML, both of which take the same prefix approach.

        What authors require, is for a string token "foaf:name" to be
        paired correctly with a distinct URI.

        The prefix based method does this pairing by splitting "foaf:name"
        on the colon, resolving "foaf" to a string, and then concatenating
        "name" to that string.

        In other words, they don't get "foaf:name" paired to a URI, they
        get "foaf" paired to another string. This is the indirection being
        referred to by Hickson (I believe).

        The drawbacks of this approach are:
        - non existent properties can easily be referenced
        "foaf:foobarbaz" will still expand to a URI
        - missing prefix/xmlns declarations result in no URI being
        created. (example: people copy and paste RDFa snippets, missing
        the xmlns or prefix declaration, and consequently the copied
        RDFa doesn't produce RDF triples, or worse, incorrect triples.)

        Additionally, there could be an issue in that XML Namespaces are not
        URIs, they are a pair of (namespace, token), and that no
        specification defines the concatenation of namespace to token in
        order to produce a single string URI - I certainly can't find it
        anyway, Harry Halpin wrote this up well here:
        http://xml.coverpages.org/HHalpinXMLVS-Extreme.html

        The requirements for RDFa authors are:
        to be able to use "namespaced" terms, there are too many
        vocabularies to be able to refer to a property as simply
        "name" and authors require to be able to say "foaf:name"
        or "bar:name" in their RDFa.
        to be able to write "foaf:name" and have it paired to a URI.

        The requirements for HTML consumers are:
        to be able to treat all properties as simple string tokens

        The requirements for RDF(a) consumers are:
        to be able to pair tokens back up to the correct URIs.
        to reference those URIs by their own preferred string tokens.

        Related RDFa 1.1 features

        @vocab and terms
        RDFa 1.1 introduces a new method, whereby a URI can be defined on an
        element, and simple terms (not containing a colon) can be used as
        properties:
        <p vocab="http://example.org/foo#">
        <a href="baz.html" rel="bar">something</a>
        </p>
        The full URI of the property is produced by concatenating the term
        (in @rel "bar") to the vocab string:
        http://example.org/foo#bar

        This addresses all issues other than:
        1- the case where you have two properties which are both "name"
        ("foaf:name" and "foo:name")
        2- the case where authors want to "mash" a set of vocabularies
        together to have their own suite of terms (profiles allow this)

        Personally (stressing personally):
        From someone who understands the space quite well, I believe
        Hicksons concerns are valid, and that neither change proposal
        properly addresses the issues and requirements of both parties
        suitably. Hicksons proposal sticks with how things are in HTML.
        Inksters proposal sticks with how things are in RDF. Neither
        proposal merges the RDF and HTML worlds suitably / perfectly.
        The introduction of profiles (in the current form) only adds more
        issues.
        The introduction of @vocab and terms partially addresses the issues.
        Microdata caters for some of the HTML needs.
        RDFa caters for most of the RDF needs
        Neither is an optimal solution for the combined RDF+HTML+Metadata
        space.

        I do not have a change proposal at this time (and would be wary of
        introducing a third option in to the mix that conflicts with both
        the stances of the HTML and RDFa WGs), but feel that the
        combination of "vocab" and "profile" should be considered to address
        issue 2, and that curies/xmlns/prefixes could be abandoned in favour
        of terms which could include colons to address issue 1 and the long
        standing issues of CURIEs, xmlns and prefix based indirection.

        Hope that helps a bit,

        Best,

        Nathan



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      • misha.wolf@thomsonreuters.com
        fyi ... From: www-tag-request@w3.org [mailto:www-tag-request@w3.org] On Behalf Of Henry S. Thompson Sent: 03 February 2011 13:54 To: nathan@webr3.org Cc:
        Message 3 of 3 , Feb 3, 2011
          fyi

          -----Original Message-----
          From: www-tag-request@... [mailto:www-tag-request@...] On Behalf
          Of Henry S. Thompson
          Sent: 03 February 2011 13:54
          To: nathan@...
          Cc: www-tag@...
          Subject: Re: Short briefing/background doc't regarding RDFa, prefixes
          and HTML

          -----BEGIN PGP SIGNED MESSAGE-----
          Hash: SHA1

          Nathan writes:

          > You've missed @profile which allows the importing of prefix and
          > term definitions from an external profile (read: hosted on the web
          > somewhere), also provides a default RDFa processor profile (meaning
          > some prefixes are always supported and mapped to a specific uri),
          > and also means that if a specified profile can't be dereferenced and
          > read correctly (yes at parse / process time) then the element on
          > which the @profile is declared, and all child elements, are skipped
          > (no triples will be extracted from these elements).

          Thanks, I thought I had worked back from step 4 of 7.5 to all the
          places URI mappings could come from, but I obviously missed @profile.
          I've updated the document [1] to reflect this.

          ht

          [1] http://www.w3.org/2001/tag/doc/RDFa_HTML_prefix_issue.html
          - --
          Henry S. Thompson, School of Informatics, University of Edinburgh
          10 Crichton Street, Edinburgh EH8 9AB, SCOTLAND -- (44) 131
          650-4440
          Fax: (44) 131 651-1426, e-mail: ht@...
          URL: http://www.ltg.ed.ac.uk/~ht/
          [mail from me _always_ has a .sig like this -- mail without it is
          forged spam]
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