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1134FW: Short briefing/background doc't regarding RDFa, prefixes and HTML

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  • misha.wolf@thomsonreuters.com
    Feb 3, 2011

      -----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

      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:

      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
      <p vocab="http://example.org/foo#">
      <a href="baz.html" rel="bar">something</a>
      The full URI of the property is produced by concatenating the term
      (in @rel "bar") to the vocab string:

      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
      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

      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,



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