RDFa

RDFa was defined in 2008 with the "RDFa in XHTML: Syntax and Processing" Recommendation.[16] Its first application was to be a module of XHTML.

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"[17]).

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 Third Edition.[18]

Is a W3C Recommendation (1.0 and 1.1) since 2009,[19] 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".[20]

In 2009 the W3C was positioned[21] to retain RDFa Lite as unique and definitive standard alternative to Microdata.[22] The position was confirmed with the publication of the HTML5 Recommendation in 2014.

2013 survey pizza charts of percentual usage,[25] showing that 79% of URLs and 43% of domains use HTML+RDFa. The average 61% (the other 39% was Microformats) is the usage indicator.

The simplified approaches to semantically annotate information items in webpages were greatly encouraged by the HTML+RDFa (released in 2008) and Microformats (since ~2005) standards.

As of 2013 these standards were encoding events, contact information, products, and so on. Despite the vCard semantics (only basic items of person and organization annotations) dominance,[25] 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 2017 show that usage[26] of HTML+RDFa is now less than that of Microformats.

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.[27] RDFaCE-Lite is a version of RDFaCE also supporting Microdata and available as a WordPress plugin.[28]

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.[29]

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>

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">
<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>

The above example can be expressed without XML namespaces in HTML5:

<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.[30]

The minimal [31] 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, it is recommended that all of these attributes are used: vocab, typeof, property; not only one of them.