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Sunday, February 24, 2008

Last Call Working Draft for RDFa in XHTML: Syntax and Processing

Members of W3C's Semantic Web Deployment Working Group and the XHTML 2
Working Group have published a Last Call Working Draft for the
specification "RDFa in XHTML: Syntax and Processing A collection of
attributes and processing rules for extending XHTML to support RDF."
RDFa is a specification for attributes to be used with languages such
as HTML and XHTML to express structured data. The rendered, hypertext
data of XHTML is reused by the RDFa markup, so that publishers don't
need to repeat significant data in the document content. This document
only specifies the use of the RDFa attributes with XHTML. The underlying
abstract representation is RDF, which lets publishers build their own
vocabulary, extend others, and evolve their vocabulary with maximal
interoperability over time. The expressed structure is closely tied to
the data, so that rendered data can be copied and pasted along with its
relevant structure. The rules for interpreting the data are generic, so
that there is no need for different rules for different formats; this
allows authors and publishers of data to define their own formats without
having to update software, register formats via a central authority, or
worry that two formats may interfere with each other. RDFa shares some
use cases with microformats. Whereas microformats specify both a syntax
for embedding structured data into HTML documents and a vocabulary of
specific terms for each microformat, RDFa specifies only a syntax and
relies on independent specification of terms (often called vocabularies
or taxonomies) by others. RDFa allows terms from multiple independently
developed vocabularies to be freely intermixed and is designed such that
the language can be parsed without knowledge of the specific term
vocabulary being used. Motivation: RDF/XML (Syntax) provides sufficient
flexibility to represent all of the abstract concepts in RDF. However,
it presents a number of challenges; first it is difficult or impossible
to validate documents that contain RDF/XML using XML Schemas or DTDs,
which therefore makes it difficult to import RDF/XML into other markup
languages. Whilst newer schema languages such as RELAX NG do provide a
way to validate documents that contain arbitrary RDF/XML, it will be a
while before they gain wide support. Second, even if one could add RDF/XML
directly into an XML dialect like XHTML, there would be significant data
duplication between the rendered data and the RDF/XML structured data.
It would be far better to add RDF to a document without repeating the
document's existing data. One motivation for RDFa has been to devise a
means by which documents can be augmented with metadata in a general
rather than hard-wired manner. This has been achieved by creating a
fixed set of attributes and parsing rules, but allowing those attributes
to contain properties from any of a number of the growing range of
available RDF vocabularies. The values of those properties are in most
cases the information that is already in an author's XHTML document.

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