|Editors||Ben Adida, Mark Birbeck|
|Related standards||RDF Schema, OWL|
RDFa (or Resource Description Framework in Attributes) is a W3C Recommendation that adds a set of attribute-level extensions to HTML, XHTML and various XML-based document types for embedding rich metadata within Web documents. The RDF data-model mapping enables its use for embedding RDF subject-predicate-object expressions within XHTML documents. It also enables the extraction of RDF model triples by compliant user agents.
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 This superseded and expanded upon the April draft; it contained rules for creating an RDFa parser, as well as guidelines for organizations wishing to make practical use of the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 1.0 reached recommendation status.
RDFa 1.1 reached recommendation status in June 2012. It differs from RDFa 1.0 in that it no longer relies on the XML-specific namespace mechanism. Therefore, it is possible to use RDFa 1.1 with non-XML document types such as HTML 4 or HTML 5. Details can be found in an appendix to HTML 5.
An additional RDFa Primer document was last updated in June 2012. (The first public Working Draft dates back to March 2006.)
Versions and variants
There are some main well-defined variants of the basic concepts, that are used as reference and as abbreviation to the W3C standards.
The HTML applications remained, "a collection of attributes and processing rules for extending XHTML to support RDF" expanded to HTML5, are now expressed in a specialized standard, the "HTML+RDFa" (the last is "HTML+RDFa 1.1 - Support for RDFa in HTML4 and HTML5").
The "HTML+RDFa" syntax of 2008 was also termed "RDFa 1.0", so, there are no "RDFa Core 1.0" standard. In general this 2008's RDFa 1.0 is used with the old XHTML standards (as long as RDFa 1.1 is used with XHTML5 and HTML5).
Is the first generic (for HTML and XML) RDFa standard, now (2015) the "RDFa Core 1.1" is in the Second Edition.
Is a W3C Recommendation (1.0 and 1.1) since 2009, as "a minimal subset of RDFa, the Resource Description Framework in attributes, consisting of a few attributes that may be used to express machine-readable data in Web documents like HTML, SVG, and XML. While it is not a complete solution for advanced data markup tasks, it does work for most day-to-day needs and can be learned by most Web authors in a day".
In 2009 the W3C was positioned to retain RDFa Lite as unique and definitive standard alternative to Microdata. The position was confirmed with the publication of the HTML5 Recommendation in 2014.
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 – a URI or CURIE specifying the resource the metadata is about
- rel and rev – specifying a relationship and reverse-relationship with another resource, respectively
- src, href and resource – specifying the partner resource
- property – specifying a property for the content of an element or the partner resource
- 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 or the partner resource (the resource that the metadata is about).
There are five "principles of interoperable metadata" met by RDFa.
- Publisher Independence – each site can use its own standards
- Data Reuse – data are 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
There is a growing number of tools for better usage of RDFa vocabularies and RDFa annotation.
As of 2013[update] these standards were encoding events, contact information, products, and so on. Despite the vCard semantics (only basic items of person and organization annotations) dominance, and some cloning of annotations along the same domain, the counting of webpages (URLs) and domains with annotations is an important statistical indicator for usage of semantically annotated information in the Web.
The statistics of 2013 show that usage of HTML+RDFa has passed the usage of Microformats (illustration), consolidating a trend of growth shown in comparisons with 2011 and 2012.
- Web-based RDFa editors
- There are already a few RDFa editors available online. RDFaCE (RDFa Content Editor) is a WYSIWYM editor based on TinyMCE to support RDFa content authoring. It supports manual and semi-automatic generation of RDFa with the support of annotation services such as DBpedia Spotlight, OpenCalais, Alchemy API, among others. RDFaCE-Lite is a version of RDFaCE also supporting Microdata and available as a WordPress plugin.
- Desktop RDFa editors
- AutôMeta is an environment for semi-automatic (or automatic) annotation of documents for publishing on the Web using RDFa. It also includes a RDFa extraction tool to provide the user with a view of the annotated triples. It is available in both CLI and GUI interfaces.
The following is an example of adding Dublin Core metadata to an XML element in an XHTML file. Dublin Core data elements are data typically added to a book or article (title, author, subject etc.)
<div xmlns:dc="http://purl.org/dc/elements/1.1/" about="http://www.example.com/books/wikinomics"> <span property="dc:title">Wikinomics</span> <span property="dc:creator">Don Tapscott</span> <span property="dc:date">2006-10-01</span> </div>
Moreover, RDFa allows the passages and words within a text to be associated with semantic markup:
<div xmlns:dc="http://purl.org/dc/elements/1.1/" about="http://www.example.com/books/wikinomics"> In his latest book <span property="dc:title">Wikinomics</span>, <span property="dc:creator">Don Tapscott</span> explains deep changes in technology, demographics and business. The book is due to be published in <span property="dc:date" content="2006-10-01">October 2006</span>. </div>
XHTML + RDFa 1.0
<?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"> <html 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"> <head> <title>John's Home Page</title> <base href="http://example.org/john-d/" /> <meta property="dc:creator" content="Jonathan Doe" /> <link rel="foaf:primaryTopic" href="http://example.org/john-d/#me" /> </head> <body about="http://example.org/john-d/#me"> <h1>John's Home Page</h1> <p>My name is <span property="foaf:nick">John D</span> and I like <a href="http://www.neubauten.org/" rel="foaf:interest" xml:lang="de">Einstürzende Neubauten</a>. </p> <p> My <span rel="foaf:interest" resource="urn:ISBN:0752820907">favorite book is the inspiring <span about="urn:ISBN:0752820907"><cite property="dc:title">Weaving the Web</cite> by <span property="dc:creator">Tim Berners-Lee</span></span></span>. </p> </body> </html>
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 Einstürzende 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:
<?xml version="1.0" encoding="UTF-8"?> <rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:foaf="http://xmlns.com/foaf/0.1/" xmlns:dc="http://purl.org/dc/elements/1.1/"> <rdf:Description rdf:about="http://example.org/john-d/"> <dc:creator xml:lang="en">Jonathan Doe</dc:creator> <foaf:primaryTopic> <rdf:Description rdf:about="http://example.org/john-d/#me"> <foaf:nick xml:lang="en">John D</foaf:nick> <foaf:interest rdf:resource="http://www.neubauten.org/"/> <foaf:interest> <rdf:Description rdf:about="urn:ISBN:0752820907"> <dc:creator xml:lang="en">Tim Berners-Lee</dc:creator> <dc:title xml:lang="en">Weaving the Web</dc:title> </rdf:Description> </foaf:interest> </rdf:Description> </foaf:primaryTopic> </rdf:Description> </rdf:RDF>
HTML5 + RDFa 1.1
The minimal  document is:
<html lang="en"> <head> <title>Example Document</title> </head> <body vocab="http://schema.org/"> <p typeof="Blog"> Welcome to my <a property="url" href="http://example.org/">blog</a>. </p> </body> </html>
that is, is recommended the use of the attributes vocab, typeof, property; not only one of them.
<html prefix="dc:http://purl.org/dc/elements/1.1/" lang="en"> <head> <title>John's Home Page</title> <link rel="profile" href="http://www.w3.org/1999/xhtml/vocab" /> <base href="http://example.org/john-d/" /> <meta property="dc:creator" content="Jonathan Doe" /> <link rel="foaf:primaryTopic" href="http://example.org/john-d/#me" /> </head> <body about="http://example.org/john-d/#me"> <h1>John's Home Page</h1> <p>My name is <span property="foaf:nick">John D</span> and I like <a href="http://www.neubauten.org/" rel="foaf:interest" lang="de">Einstürzende Neubauten</a>. </p> <p> My <span rel="foaf:interest" resource="urn:ISBN:0752820907">favorite book is the inspiring <span about="urn:ISBN:0752820907"><cite property="dc:title">Weaving the Web</cite> by <span property="dc:creator">Tim Berners-Lee</span></span></span>. </p> </body> </html>
Note how the prefix foaf is still used without declaration. RDFa 1.1 automatically includes prefixes for popular vocabularies such as FOAF.
- Microformats, a simplified approach to semantically annotate data in web pages
- Open Graph protocol, a way to use RDFa to integrate web pages into the Facebook social graph
- Microdata - another approach at embedding semantics in HTML using additional attributes
- eRDF, an alternative to RDFa (now obsolete)
- GRDDL, a way to extract (annotated) data out of XHTML and XML documents and transform it into an RDF graph
- Schema.org, search-engine supported schemas for structured data markup on web pages that can be expressed as RDFa
- "XHTML RDFa Primer". W3.org. 2013. Retrieved 14 June 2013. RDFa is defined as "Resource Description Framework in Attributes".
- RDFa community Wiki
- "XHTML and RDF W3C Note 14 February 2004". World Wide Web Consortium. 2004-02-14. Retrieved 2007-12-27.
- Semantic Web Interest Group This group is previously known as the RDF Interest Group.
- "Semantic Web Interest Group". XML.com. 2004-03-03. Retrieved 2007-12-27. External link in
- "XHTML 2.0 W3C Working Draft 22 July 2004, 19. XHTML Metainformation Attributes Module". World Wide Web Consortium. 2004-07-22. Retrieved 2007-10-06.
- "XML and Semantic Web W3C Standards Timeline" (PDF).
- "RDF/A Syntax: A collection of attributes for layering RDF on XML languages". 2004-10-11. Retrieved 2009-05-14.
- "XHTML RDFa Modules, Modules to support RDF annotation of elements, W3C Editor's Draft 2 April 2007". World Wide Web Consortium. 2007-04-02. Retrieved 2007-10-06.
- For examples of this, see: http://www.joost.com/09400ax http://weborganics.co.uk/files/hAudio-RDFa.xhtml
- "RDFa in XHTML: Syntax and Processing, A collection of attributes and processing rules for extending XHTML to support RDF, W3C Working Draft 18 October 2007". World Wide Web Consortium. 2007-10-18. Retrieved 2007-10-20.
- "RDFa in XHTML: Syntax and Processing, A collection of attributes and processing rules for extending XHTML to support RDF, W3C Recommendation 14 October 2008". World Wide Web Consortium. 2008-10-14. Retrieved 2008-10-15.
- "RDFa Core 1.1 - Syntax and processing rules for embedding RDF through attributes". World Wide Web Consortium. 2012-06-07. Retrieved 2012-08-25.
- "HTML+RDFa 1.1 - Support for RDFa in HTML4 and HTML5". World Wide Web Consortium. 2012-03-29. Retrieved 2012-08-25.
- "RDFa 1.1 Primer - Rich Structured Data Markup for Web Documents". World Wide Web Consortium. 2008-06-20. Retrieved 2012-08-25.
- "RDFa in XHTML: Syntax and Processing - A collection of attributes and processing rules for extending XHTML to support RDF", W3C Recommendation 14 October 2008. http://www.w3.org/TR/2008/REC-rdfa-syntax-20081014/
- "HTML+RDFa 1.1 - Support for RDFa in HTML4 and HTML5", W3C Recommendation 22 August 2013. http://www.w3.org/TR/html-rdfa/
- "RDFa Core 1.1 - Second Edition - Syntax and processing rules for embedding RDF through attribute", W3C Recommendation 22 August 2013. http://www.w3.org/TR/rdfa-core/
- first draft 1.1 cite ~2009 as ~year of "RDFa Lite 1.0"... Please check better reference.
- "RDFa Lite 1.1", W3C Recommendation 07 June 2012. http://www.w3.org/TR/rdfa-lite/ (second edition at 2015)
- Final W3C position (ISSUE-76), establishing that Microdata syntax simply duplicates what RDFa Lite already does.
- Building Interoperable Web Metadata
- RDFa – Implications for Accessibility
- "Web Data Commons – RDFa, Microdata, and Microformat Data Sets". section 3.1, "Extraction Results from the November 2013 Common Crawl Corpus". 2013. Retrieved 2015-02-21.
- RDFaCE home page
- RDFaCE WordPress Plugin
- AutôMeta Project Page
- "Example of an HTML+RDFa 1.1 document" at http://www.w3.org/TR/html-rdfa/#document-conformance
- "RDFa Core Initial Context - Vocabulary Prefixes". World Wide Web Consortium. 2012-05-27. Retrieved 2012-08-25.