RDFa was first proposed by Mark Birbeck in the form of a W3C note entitled XHTML and RDF, which was then presented to the Semantic Web Interest Group at the W3C's 2004 Technical Plenary. Later that year the work became part of the sixth public Working Draft of XHTML 2.0. Although it is generally assumed that RDFa was originally intended only for XHTML 2, in fact the purpose of RDFa was always to provide a way to add a metadata to any XML-based language. Indeed, one of the earliest documents bearing the RDF/A Syntax name, has the sub-title A collection of attributes for layering RDF on XML languages. The document was written by Mark Birbeck and Steven Pemberton, and was made available for discussion on October 11, 2004.
In April 2007 the XHTML 2 Working Group produced a module to support RDF annotation within the XHTML 1 family. As an example, it included an extended version of XHTML 1.1 dubbed XHTML+RDFa 1.0. Although described as not representing an intended direction in terms of a formal markup language from the W3C, limited use of the XHTML+RDFa 1.0 DTD did subsequently appear on the public Web.
October 2007 saw the first public Working Draft of a document entitled RDFa in XHTML: Syntax and Processing. This superseded and expanded upon the April draft; it contained rules for creating an RDFa parser, as well as guidelines for organizations wishing to make practical use of the technology.
In October 2008 RDFa reached Recommendation status.
An additional RDFa Primer document was last updated in June 2008. (The first public Working Draft dates back to March 2006.)
The essence of RDFa is to provide a set of attributes that can be used to carry metadata in an XML language (hence the 'a' in RDFa).
These attributes are:
about and src a URI or CURIE specifying the resource the metadata is about
rel and rev specifying a relationship or reverse-relationship with another resource
href and resource specifying the partner resource
property specifying a property for the content of an element
content optional attribute that overrides the content of the element when using the property attribute
datatype optional attribute that specifies the datatype of text specified for use with the property attribute
typeof optional attribute that specifies the RDF type(s) of the subject (the resource that the metadata is about).
Benefits of RDFa
Five "principles of interoperable metadata" met by RDFa.
Publisher Independence each site can use its own standards
Data Reuse data is not duplicated. Separate XML and HTML sections are not required for the same content.
Self Containment The HTML and the RDF are separated
Schema Modularity The attributes are reusable
Evolvability additional fields can be added and XML transforms can extract the semantics of the data from an XHTML file
Moreover, RDFa allows the passages and words within a text to be associated with semantic markup:
xmlns:dc="http://purl.org/dc/elements/1.1/" about="http://www.example.com/books/wikinomics"> In his latest book property="dc:title">Wikinomics, property="dc:creator">Don Tapscott explains deep changes in technology, demographics and business. The book is due to be published in property="dc:date" content="2006-10-01">October 2006.
XHTML+RDFa 1.0 example
The following is an example of a complete XHTML+RDFa 1.0 document. It uses Dublin Core and FOAF, an ontology for describing people and their relationships with other people and things:
<?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML+RDFa 1.0//EN" "http://www.w3.org/MarkUp/DTD/xhtml-rdfa-1.dtd"> xmlns="http://www.w3.org/1999/xhtml" xmlns:foaf="http://xmlns.com/foaf/0.1/" xmlns:dc="http://purl.org/dc/elements/1.1/" version="XHTML+RDFa 1.0" xml:lang="en"> John's Home Page href="http://example.org/john-d/" /> property="dc:creator" content="Jonathan Doe" /> rel="foaf:primaryTopic" href="http://example.org/john-d/#me" /> about="http://example.org/john-d/#me"> John's Home PageMy name is property="foaf:nick">John D and I like href="http://www.neubauten.org/" rel="foaf:interest" xml:lang="de">Einst rzende Neubauten. My rel="foaf:interest" resource="urn:ISBN:0752820907">favorite book is the inspiring about="urn:ISBN:0752820907"> property="dc:title">Weaving the Web by property="dc:creator">Tim Berners-Lee
In the example above, the document URI can be seen as representing an HTML document, but the document URI plus the "#me" string http://example.org/john-d/#me represents the actual person, as distinct from a document about them. The foaf:primaryTopic in the header tells us a URI of the person the document is about. The foaf:nick property (in the first span element) contains a nickname for this person, and the dc:creator property (in the meta element) tells us who created the document. The hyperlink to the Einst rzende Neubauten website contains rel="foaf:interest", suggesting that John Doe is interested in this band. The URI of their website is a resource.
The foaf:interest inside the second p element is referring to a book by ISBN number. The resource attribute defines a resource in a similar way to the href attribute, but without defining a hyperlink. Further into the paragraph, a span element containing an about attribute defines the book as another resource to specify metadata about. The book title and author are defined within the contents of this tag using the dc:title and dc:creator properties.
Here are the same triples when the above document is automatically converted to RDF/XML: