W3C

RDFa Syntax

A collection of attributes for layering RDF on XML languages

W3C Editor's Draft 30 August 2007

This version:
http://www.w3.org/MarkUp/2007/ED-rdfa-syntax-20070830
Latest version:
http://www.w3.org/TR/rdfa-syntax
Previous Editor's Draft:
http://www.w3.org/MarkUp/2007/ED-rdfa-syntax-20070812
Diff from previous Editor's Draft:
rdfa-syntax-diff.html
Editors:
Mark Birbeck, x-port.net Ltd. Mark.Birbeck@x-port.net
Steven Pemberton, CWI
Ben Adida, Creative Commons ben@adida.net
Shane McCarron, Applied Testing and Technology, Inc. shane@aptest.com

This document is also available in these non-normative formats: PostScript version , PDF version , ZIP archive , and Gzip'd TAR archive .

The English version of this specification is the only normative version. Non-normative translations may also be available.

Copyright © 2007 W3C ® ( MIT , ERCIM , Keio ), All Rights Reserved. W3C liability , trademark and document use rules apply.

Abstract

Current web pages, written in HTML, contain significant inherent structured data. When publishers can express this data more completely, and when tools can read it, a new world of user functionality becomes available, letting users transfer structured data between applications and web sites. An event on a web page can be directly imported into a user's desktop calendar; a license on a document can be detected so that users can be informed of their rights automatically; a photo's creator, camera setting information, resolution, and topic can be published as easily as the original photo itself, enabling structured search and sharing.

RDFa is a syntax for expressing this structured data in XHTML. The rendered, hypertext data of XHTML is reused by the RDFa markup, so that publishers don't repeat themselves. 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 do not have to be different rules for different structures; this allows authors and publishers of data to define their own formats without having to update software, or register formats via a central authority.

This document is a detailed syntax specification for RDFa. For a more gentle introduction, please consult the RDFa Primer .

Status of this Document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

This is an internal draft produced by the Semantic Web Deployment Working Group [SWD-WG] , in cooperation with the XHTML 2 Working Group [HTML-WG] . Initial work on RDFa began with the Semantic Web Best Practices and Deployment Working Group [SWBPD-WG] .

This document has no official standing within the W3C. It is also a work in progress, which means it may change at any time, without warning, and you shouldn't rely on anything in this document.

Table of Contents

1. Motivation

RDF/XML [RDF-SYNTAX] provides sufficient flexibility to represent all of the abstract concepts in RDF [RDF-CONCEPTS] . However, it presents two challenges; first it is difficult or impossible to validate documents that contain RDF/XML using XML Schemas or DTD's, which makes it difficult to import RDF/XML into other markup languages. Whilst newer schema languages such as RELAX NG [RELAXNG] 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. For example, an XHTML document that explicitly renders its author's name "Mark Birbeck" should not need to repeat this name for RDF expression of the same concept: the existing markup should be augmentable to RDF with minimal data repetition.

Third, as users often want to transfer structured data from one application to another, sometimes to or from a non-web-based application, it is highly beneficial to express the web data's structure "in context." A user can then get contextual information about specific rendered data, for example by "right-clicking" on an item of interest.

In the past, some attributes were 'hard-wired' directly into the markup language to represent specific concepts. For example, in XHTML 1.1 and HTML there is a cite attribute. The attribute allows an author to add information to a document to indicate the origin of a quote.

The motivation of RDFa is to devise a means by which documents can be augmented with metadata in a generic rather than hard-wired manner, using property values from the growing range of available taxonomies, reusing existing content from the host language.

This specification takes the pressure off language authors to anticipate all the structural requirements users of their language might have by outlining a new XML syntax for RDF that relies only on XML attributes, and so can be easily imported into other markup languages allowing them to carry arbitrary RDF.

2. Terms and Abbreviations

2.1. Namespaces

In the following examples, for brevity assume that the following namespace prefixes are defined:

cc: http://creativecommons.org/ns#
dc: http://purl.org/dc/elements/1.1/
ex: http://example.org/
foaf: http://xmlns.com/foaf/0.1/
rdf: http://www.w3.org/1999/02/22-rdf-syntax-ns#
rdfs: http://www.w3.org/2000/01/rdf-schema#
svg: http://www.w3.org/2000/svg
xh11: http://www.w3.org/1999/xhtml
xsd: http://www.w3.org/2001/XMLSchema#
biblio: http://example.org/biblio/0.1
taxo: http://purl.org/rss/1.0/modules/taxonomy/

2.2. RDF Terminology

This document uses the following terminology defined in [RDF-CONCEPTS] :

We also add two further concepts, an [RDFa element] and the [context statement] both of which are explained in the processing section.

The aim of RDFa is to allow [RDF graph]s to be carried in XML documents of any type. An [RDF graph] comprises [node]s linked by relationships. The basic unit of a graph is a [triple], in which a subject [node] is linked to an object [node] via a [predicate]. The subject [node] is always either an [RDF URI reference] or a [blank node], the predicate is always an [RDF URI reference], and the object of a statement can be an [RDF URI reference], a [literal], or a [blank node].

In RDFa, a subject [RDF URI reference] is indicated using the attribute about and predicates are represented using one of the attributes property , rel , or rev . Objects which are [RDF URI reference]s are represented using the attribute href , whilst objects that are [literal]s are represented either with the attribute content , or the content of the element in question.

2.2.1. N-Triples

Most of the examples in this document are shown translated into N-Triples [N-TRIPLES] syntax, with a slight variation in that QNames can be used to replace a URI reference. To tell them apart, the QName will have no angle brackets, e.g. the triple:

Example 

<http://internet-apps.blogspot.com/> dc:creator "Mark Birbeck" .

should be read as an abbreviation for the N-Triples syntax:

Example 

<http://internet-apps.blogspot.com/>
  <http://purl.org/dc/elements/1.1/creator>
  "Mark Birbeck" .

Datatypes can also be abbreviated, so the following:

Example 

<> dc:title
   "E = mc<sup>2</sup>: The Most Urgent Problem of Our Time"^^rdf:XMLLiteral .

should be read as an abbreviation for this N-Triples statement:

Example 

<> <http://purl.org/dc/elements/1.1/creator>
   "E = mc<sup>2</sup>: The Most Urgent Problem of Our Time"^^<http://www.w3.org/1999/02/22-rdf-syntax-ns#XMLLiteral> .

2.3. Using xml:base

All [RDF URI references] are subject to xml:base [XML-BASE] . Note that this means that in the absence of an xml:base attribute, the document containing the RDF statements is itself the base.

An example follows to show how xml:base affects the subject:

Example 

<span xml:base="http://internet-apps.blogspot.com/">
    <link about="" rel="dc:creator" href="http://www.blogger.com/profile/1109404" />
    <meta about="" property="dc:title" content="Internet Applications" />
</span>

The triples generated would be as follows:

Example 

<http://internet-apps.blogspot.com/>
   dc:creator 
   <http://www.blogger.com/profile/1109404> .
<http://internet-apps.blogspot.com/>
   dc:title
   "Internet Applications" .

2.4. Compact URIs

In order to allow for the compact expression of RDF statements, RDFa uses a variant of QNames [QName] that allows the contraction of all URIs (QNames have a syntactic restriction on the sorts of URI that can be contracted).

These Compact URIs are called CURIEs here.

The rel , rev , and property attributes accept CURIE-only datatypes, while href and about accept mixed CURIE/URI data. In particular, the following notation is a valid RDFa statement:

Example 

This document is licensed under a 
<a xmlns:cclicenses="http://creativecommons.org/licenses/"
   rel="cc:license"
   href="[cclicenses:by/nc-nd/3.0/]">
  Creative Commons License
</a>.

which generates the following triple, as expected:

Example 

<> cc:license <http://creativecommons.org/licenses/by/nc-nd/3.0/> .

2.4.1. CURIE Syntax Definition

Note that this syntax definition will ultimately be defined in an external document [ CURIE ].

A CURIE is comprised of two components, a prefix and a reference . The prefix is separated from the reference by a colon ( : ). To disambiguate a CURIE when it appears in a context where a normal [ URI ] may also be used, the entire CURIE is permitted to be enclosed in brackets ( [ , ] ). 


    curie       :=   basic_curie | safe_curie
    safe_curie  :=   '[' basic_curie ']'
    basic_curie :=   [ prefix ] ':' reference
    prefix      :=   NCName
    reference   :=   irelative-ref (as defined in [IRI])

When CURIES are used in an XML-based host language, prefix values MUST be able to be defined using the 'xmlns:' syntax specified in [ XMLNAMES ]. Such host languages MAY also provide additional prefix mapping definition mechanisms.

When CURIES are used in a non-XML host language, the host language MUST provide a mechanism for defining the mapping from the prefix to an IRI.

A host language MAY define a mechanism for defining a default prefix value. In such a host language, if the prefix is omitted from a CURIE, the default prefix value is used.

The concatenation of the prefix associated with a CURIE and its reference MUST be an IRI [ IRI ] .

The CURIE prefix '_' is reserved. For this reason, prefix declarations using '_' SHOULD be avoided by authors.

Host languages MAY define additional constraints on these syntax rules when CURIES are used in the context of those host languages. Host languages MUST NOT relax the CURIE syntax constraints defined this specification.

3. Introduction to the structure of RDFa

3.1. Overview

The main idea behind the syntax for RDFa is that existing data should be easy to update to convey RDF triples. Thus, the bulk of RDFa can be expressed using only attributes applied to existing elements within the XML document. These are

about ,
a URI or CURIE, used for stating what the data is about (the 'subject' in RDF terminology)
rel ,
a list of CURIEs, used for expressing relationships between two resources (a 'predicate' in RDF)
rev ,
a list of CURIEs, used for expressing reverse relationships between two resources (also a 'predicate')
property ,
a list of CURIEs, used for expressing relationships between the subject and some literal text (also a 'predicate')
href ,
a URI or CURIE, for expressing the partner resource of a relationship (the 'object' in RDF)
resource ,
a URI or CURIE, for expressing the partner resource of a relationship that is not intended to be 'clickable' (also an 'object')
src ,
a URI or CURIE, for expressing the partner resource of a relationship when the resource is embedded (also an 'object')
datatype ,
a schema datatype, to express the datatype of a literal
content
a string, for supplying alternative, machine-readable content for a literal.

As you will remark, several of these attributes are borrowed directly from HTML, and have largely the same meaning as in HTML.

For example, given an XHTML chunk as follows:

Example 

This photo was taken by
<span class="author">Mark Birbeck</span>.

a simple attribute augmentation can yield an RDF triple: 

This photo was taken by
<span class="author" about="photo1.jpg" property="dc:creator">Mark Birbeck</span>.

which yields:

Example 

<photo1.jpg> dc:creator "Mark Birbeck"^^rdf:XMLLiteral .

Similarly, links can be augmented to express RDF triples. Consider an XHTML chunk:

Example 

This photo was taken by
<a href="http://www.blogger.com/profile/1109404">Mark Birbeck</a>.

When the RDF object is a URI, the RDF predicate is designated using rel : 

This photo was taken by
<a about="photo1.jpg" rel="dc:creator"

   href="http://www.blogger.com/profile/1109404">Mark Birbeck</a>.

which yields:

Example 

<photo1.jpg> dc:creator <http://www.blogger.com/profile/1109404> .

It's important to note that the various RDFa attributes can be used on any existing element of the XML dialect. Note also that one can express a reverse relationship using the rev attribute. For example, if the photo in question is actually a depiction of Mark, one could write: 

This photo was taken by
<a about="photo1.jpg" rev="foaf:img"

   href="http://www.blogger.com/profile/1109404">Mark Birbeck</a>.

which would yield:

Example 

<http://www.blogger.com/profile/1109404> foaf:img <photo1.jpg> .

Both relations can be expressed simultaneously: 

This photo was taken by
<a about="photo1.jpg" rel="dc:creator" rev="foaf:img"

   href="http://www.blogger.com/profile/1109404">Mark Birbeck</a>.

which then yields both triples:

Example 

<photo1.jpg> dc:creator <http://www.blogger.com/profile/1109404> .
<http://www.blogger.com/profile/1109404> foaf:img <photo1.jpg> .

It's possible to go further and add the attributes used for denoting statements in which the object is a [literal]: 

This photo was taken by
<a about="photo1.jpg" property="dc:title"
   rel="dc:creator"
   rev="foaf:img" href="http://www.blogger.com/profile/1109404">Mark Birbeck</a>.

which would then yield:

Example 


<photo1.jpg> dc:creator <http://www.blogger.com/profile/1109404> .
<http://www.blogger.com/profile/1109404> foaf:img <photo1.jpg> .
<photo1.jpg> dc:title "Mark Birbeck" .

Or going further: 

This photo was taken by
<a about="photo1.jpg" property="dc:title"
   content="Portrait of Mark" rel="dc:creator"

   rev="foaf:img" href="http://www.blogger.com/profile/1109404">Mark Birbeck</a>.

which would then yield:

Example 

<photo1.jpg> dc:creator <http://www.blogger.com/profile/1109404> .
<http://www.blogger.com/profile/1109404> foaf:img <photo1.jpg> .
<photo1.jpg> dc:title "Portrait of Mark" .

It's possible to do all of this without ambiguity, since the property attribute always denotes a predicate in a statement in which the object is a [literal], whilst the rel and rev attributes always denote a predicate in a statement in which the object is a [URI reference]. Put a different way, property always works with content , whilst rel and rev work with href or resource .

Of course, the more natural way to express the three above triples is to strive to make all metadata literals and URIs meaningful within the host XML dialect. Specifically, in the case of XHTML2, it makes sense to render as much of the useful metadata as possible and use RDFa to mark up this rendered data. The following XHTML thus generates the same triples shown above.

Example 

This photo, entitled 
<span about="photo1.jpg" property="dc:title">Portrait of Mark</span>
was taken by
<a about="photo1.jpg" rel="dc:creator" rev="foaf:img"
   href="http://www.blogger.com/profile/1109404">Mark himself</a>.

The value of the about attribute is inherited from parent elements. The following XHTML thus generates the very same triples as the XHTML above.

Example 

<div about="photo1.jpg">
  This photo, entitled
  <span property="dc:title">Portrait of Mark</span>
  was taken by
  <a rel="dc:creator" rev="foaf:img" 
     href="http://www.blogger.com/profile/1109404">Mark himself</a>.
</div>

3.2. Qualifying document components

A second feature of RDFa is that it is possible to use parts of the host document to provide the [subject] of a [triple]. This marks RDFa from other approaches to serialising RDF, in that the the same syntax can now be used to make statements about parts of a document, and external documents.

It is possible to make such statements using the syntax introduced in the examples above:

Example 

<html xmlns:dc="http://purl.org/dc/elements/1.1/">
    <head>
        <title>On <em>Crime and Punishment</em></title>
    </head>
    <body>
        <blockquote id="q1" about="#q1" rel="dc:source" resource="urn:isbn:0140449132" >

            <p>
                Rodion Romanovitch! My dear friend! If you go on in this way
                you will go mad, I am positive! Drink, pray, if only a few drops!
            </p>
        </blockquote>
    </body>
</html>

Note that the use of an href attribute on an element does not necessarily make it clickable, nor does it affect the display of the element (such as underlining it in blue). These are decisions made by the host language.

3.3. Relating document components

Using qualifying statements, RDFa allows a single XML dialect document to include multiple RDF entities. Relations between the various entities of a given page can also be defined using RDFa notation.

Consider the following XHTML, which defines two RDF entities of type taxo:topic , two RDF entities of type biblio:Publication , metadata pertinent to each publication, including dc:title and dc:creator , and relations of type taxo:topics between the publications and tags:

Example 

<html xmlns:dc="http://purl.org/dc/elements/1.1/">
    <head>
        <title>Mark's Publications</title>
    </head>
    <body>
       <h2>Tags</h2>
       <div id="tag_standards">
          <link rel="rdf:type" href="[taxo:topic]" />
          Standards
       </div>
       <div id="tag_xforms">
          <link rel="rdf:type" href="[taxo:topic]" />
          XForms
       </div>
       <h2>Publications</h2>
       <div id="publication_1">
         <link rel="rdf:type" href="[biblio:Publication]" />
         <link rel="dc:creator" href="http://www.blogger.com/profile/1109404" />
         <meta property="dc:title">A Standards-Based Virtual Machine</meta>
         <link rel="taxo:topics" href="#tag_standards" />
       </div>
       <div id="publication_2">
         <link rel="rdf:type" href="[biblio:Publication]" />
         <link rel="dc:creator" href="http://www.blogger.com/profile/1109404" />
         <meta property="dc:title">XForms and Internet Applications</meta>
         <link rel="taxo:topics" href="#tag_standards" />
         <link rel="taxo:topics" href="#tag_xforms" />
       </div>
    </body>
</html>

This yields the expected triples:

Example 

<#tag_standards> rdf:type taxo:topic .
<#tag_xforms> rdf:type taxo:topic .
<#publication_1> rdf:type biblio:Publication .
<#publication_1> dc:creator <http://www.blogger.com/profile/1109404>
<#publication_1> dc:title "A Standards-Based Virtual Machine"^^rdf:XMLLiteral .
<#publication_1> taxo:topics <#tag_standards> .
<#publication_2> rdf:type biblio:Publication .
<#publication_2> dc:creator <http://www.blogger.com/profile/1109404>
<#publication_2> dc:title "XForms and Internet Applications"^^rdf:XMLLiteral .
<#publication_2> taxo:topics <#tag_standards> .
<#publication_2> taxo:topics <#tag_xforms> .

Beyond this theoretical example, this application of RDFa is particularly useful for formats like FOAF. (See examples.)

3.4. Global RDF statements

The previous series of examples may mislead one to think that RDFa statements are only contextual, only meant to qualify existing elements. However, as the first examples implied, a fixed about attribute can be used to specify a global subject. It is actually quite easy to make independent, global RDF statements. Statements like:

Example 

This document is licensed under a 
<a about="" rel="cc:license" 
   href="http://creativecommons.org/licenses/by-nc-nd/3.0/">
  Creative Commons
</a>.

will produce the same triple no matter where they're located in the document:

Example 

<> cc:license <http://creativecommons.org/licenses/by-nc-nd/3.0/> .

4. RDFa in detail

4.1. Processing

An [RDFa element] is defined as any XML element that contains one or more RDFa attributes about , property , rel , rev , href , src , resource , content (but not datatype ).

Processing proceeds by examining each [RDFa element] in turn. The [RDFa element] under consideration at any time is called the [current statement], and its parent element (which does not need to be an [RDFa element) is called the [context statement].

As each [RDFa element] is examined, the processor establishes the RDF triples it generates. Each of the attributes rel , rev , and property expresses one or more triples: each accepts space-separated values, and each such value the predicate for one triple. The other attributes are used to express the subject and object of the triples.

4.2. Compact URIs

When the value of an attribute is of type CURIE/URI [REF], it can either be a URI, absolute or relative, or a CURIE in square brackets. Some example values are:

@@ The syntax is prefix ":" Tail, where Tail is any tail of the production for IRI refs [ref]. URIs are constructed by concatenating the URI associated with the prefix and the Tail. The result MUST be a valid IRI ref.

4.3. Establishing the subject

4.3.1. The about attribute

In most cases, the subject of a statement is determined by the about attribute, on the element itself, or otherwise from the closest ancestor of that element that has an about attribute.

There is one exception: if a closer ancestor element includes a rel or rev attribute with no href , src or resource , then the subject is the CURIE/URI that corresponds to that parent element (@@ as described later under object resolution. @@ will be moved here)

If neither type of subject ancestor is found then the subject is the current document.

Example 


Daniel knows
<a about="mailto:daniel.brickley@bristol.ac.uk"
   rel="foaf:knows" href="mailto:libby.miller@bristol.ac.uk">Libby</a>.
   
Libby knows
<a about="mailto:libby.miller@bristol.ac.uk"
   rel="foaf:knows" href="mailto:ian.sealy@bristol.ac.uk">Daniel</a>.

Example 


<div about="photo1.jpg">
  <span class="attribution-line">this photo was taken by
    <span property="dc:creator">Mark Birbeck</span>
  </span>
</div>

will inherit the about attribute from the enclosing div and yield the expected triple:

Example 


<photo1.jpg> dc:creator "Mark Birbeck"^^rdf:XMLLiteral .

@@ Example of using rel and rev to establish the subject

4.4. Establishing the predicate

The predicate of a statement is specified using a property , rel or rev attribute. These attributes can be placed on any element in a document, can co-exist on the same element. Each of these attributes accepts space-separated CURIEs, each of which expresses exactly one triple. The attribute indicates the type of resolution to use for the subject and object of the triple. For simplicity, we assume an attribute value of a single CURIE. We explain the space-separated multiple-values situation later.

4.4.1. The property attribute

A property attribute designates a predicate whose object is a literal. The object of the triple is determined using [literal] object resolution (Section 4.4). The subject of the triple is determined using subject resolution (Section 4.3). The following example indicates the name of the author responsible for the text being quoted:

Example 

<blockquote about="#q1">
    <p>
        Rodion Romanovitch! My dear friend! If you go on in this way
        you will go mad, I am positive! Drink, pray, if only a few drops!
    </p>
    <p>
        by <span property="dc:creator">Fyodor Dostoevsky</span>
    </p>
</blockquote>

4.4.2. The rel attribute

A rel attribute designates a predicate whose object is a non-literal. The subject of the triple is determined using subject resolution (Section 4.3). The object of the triple is determined using [URI reference] object resolution (Section 4.4). The following example indicates that one 'FOAF person' knows another:

Example 

Daniel <a about="mailto:daniel.brickley@bristol.ac.uk"
          rel="foaf:knows" href="mailto:libby.miller@bristol.ac.uk">knows</a> Libby.

The triple generated is:

Example 

<mailto:daniel.brickley@bristol.ac.uk>
  foaf:knows
  <mailto:libby.miller@bristol.ac.uk> .

4.4.3. The rev attribute

A rev attribute, like its cousin the rel attribute, indicates a predicate whose object is a non-literal, though its subject and object resolutions are reversed. The subject of the triple is determined using [URI reference] object resolution (Section 4.4). The object of the triple is determined using subject resolution (Section 4.3). Note that resolution is effectively the same as if the rev attribute had been a rel attribute with object and subject reversed. The following example indicates that one 'FOAF person' knows another:

Example 

<a about="mailto:daniel.brickley@bristol.ac.uk"
   rev="foaf:knows" href="mailto:libby.miller@bristol.ac.uk">Libby</a> knows Daniel.

and the [triple] generated is essentially a reversal of our previous example:

Example 

<mailto:libby.miller@bristol.ac.uk>
  foaf:knows
  <mailto:daniel.brickley@bristol.ac.uk> .

4.4.4. Using both rel and rev attribute

It is perfectly acceptable to use both rel and rev attributes within the same element. Predictably, this approach yields two triples, without repeating the subject and object. For example:

Example 

<a about="mailto:daniel.brickley@bristol.ac.uk" 
   rel="foaf:knows" rev="foaf:knows"
   href="mailto:libby.miller@bristol.ac.uk" >Libby</a> and Daniel know each other.

expresses:

Example 

<mailto:libby.miller@bristol.ac.uk>
  foaf:knows 
  <mailto:daniel.brickley@bristol.ac.uk> .
<mailto:daniel.brickley@bristol.ac.uk> 
  foaf:knows
  <mailto:libby.miller@bristol.ac.uk> .

The predicates need not be the same, of course.

4.4.5. Multiple Attribute Values

The rel , rev , and property attributes accept multiple space-separated CURIEs as a single attribute value. When there is more than one CURIE, then each expresses the exact same triples it would if it were the single CURIE in the attribute value. For example:

Example 

This document was authored and published by
<a about="" rel="dc:creator dc:publisher" rel="http://example.org/~markb">
    Mark Birbeck
</a>.

is interpreted by performing the normal subject and object resolutions dictated by the rel attribute on both the dc:creator and dc:publisher values. The resulting triples are:

Example 

<> dc:creator <http://example.org/~markb> .
<> dc:publisher <http://example.org/~markb> .

The same exact reasoning applies to the rev and property attributes.

4.5. Establishing the object

The object of statement can be set using one of the attributes content or href . Which attribute is used depends on how the predicate is indicated. If the predicate is set using property then the object is a [literal], and its value is given by the content attribute or the element content. If the predicate is set with the rel attribute, then the object is a non-literal whose value depends on the presence and value of href attribute. If the predicate is expressed with the rev attribute, then the object will be obtained using subject resolution as defined in the next section, while this section explains how to set that predicate's subject .

4.5.1. Literal object resolution using the content attribute

The content attribute can be used to indicate a [plain literal] as follows:

Example 

<meta about="http://internet-apps.blogspot.com/"
      property="dc:creator" content="Mark Birbeck" />

or, alternatively, using the content of the element ( meta or other) as an [XMLliteral]:

Example 

<span about="http://internet-apps.blogspot.com/"
      property="dc:creator">Mark Birbeck</span>

If the element that carries the property attribute also carries a content attribute and is non-empty, the value of the content attribute takes precedence and is taken to be the object of the triple. More details on determining the type of a literal object are provided in Section 5.1.

4.5.2. URI object resolution using the href attribute

When a triple predicate has been expressed using the rel attribute, the href attribute on the [RDFa statement]'s element is used to indicate the object as a [URI reference]. Its type, just like that of the about attribute, is CURIE/URI:

Example 

<link about="mailto:daniel.brickley@bristol.ac.uk"
      rel="foaf:knows" href="mailto:libby.miller@bristol.ac.uk" />

4.5.3. rel without href

When a triple predicate has been expressed using the rel attribute, and no href attribute exists on the same [RDFa element], then the CURIE/URI represented by this element is used as the object. Specifically, this CURIE/URI is affected by the about and id attributes. When neither is present, the object is a bnode (bnodes are discussed further in Section 5 [REF]). In all cases, the subject resolution for child elements is affected: where they do not override the subject, their subject is this same CURIE/URI here resolved as the object.

Consider, for example, a simple fragment of HTML for describing the creator of a web page, with further information about the creator, including his name and email address:

Example 

<div rel="dc:creator">
  <span property="foaf:name">Ben Adida</span>
  (<a property="foaf:mbox" href="mailto:ben@adida.net">ben@adida.net</a>)

The above yields the following triples:

Example 

<> dc:creator _:div0 .
_:div0 foaf:name "Ben Adida" .
_:div0 foaf:mbox <mailto:ben@adida.net> .

4.6. Summary

This section will contain a summary of the syntax.

5. RDF Concepts

Having established the different parts of the syntax of RDFa, we will now look at the various aspects of the RDF Abstract Syntax, and see how they can be represented in RDFa.

5.1. Literals as Objects

When a property predicate is used in RDFa, the object is expected to be a literal. This literal can be optionally typed by a datatype attribute within the same RDFa element. The absence or presence of this attribute has a significant impact on the interpretation of the literal.

5.1.1. Without datatype

Without a datatype attribute, the object literal will either be a plain literal or an XML literal, depending on whether the content attribute is used. For example, consider the following XHTML with RDFa which designates the author of a web page:

Example 

<html xmlns="http://www.w3.org/1999/xhtml">
    <head property="dc:creator" content="Mark Birbeck">
        <title>Internet Applications</title>
    </head>
...

In this case, with the use of the content attribute indicates that the object is a plain literal:

Example 

<> dc:creator "Mark Birbeck" .

On the other hand, the following RDFa will yield a slightly different triple:

Example 

<html xmlns="http://www.w3.org/1999/xhtml">
    <head>
        <title>Internet Applications</title>
    </head>
    <body>
    <span property="dc:creator">Mark Birbeck</span>
    </body>
...

which yields an XML literal object:

Example 

<> dc:creator "Mark Birbeck"^^rdf:XMLLiteral .

5.1.1.1. XML Literals

The default rdf:XMLLiteral type plays a significant role. XML documents cannot contain XML mark-up in their attributes, which means it is not possible to represent XML within the content attribute. The following would cause an XML parser to generate an error:

Example 

<head about="">
    <meta property="dc:title"
      content="E = mc<sup>2</sup>: The Most Urgent Problem of Our Time" />
</head>

It does not help to escape the content, since the output would simply be a string of text containing numerous ampersands:

Example 

<> dc:title
   "E = mc&amp;amp;lt;sup&amp;amp;gt;2&amp;amp;lt;/sup&amp;amp;gt;: The Most Urgent Problem of Our Time" .

RDF does, however, provide a datatype for indicating [XML literal]s. RDFa therefore adds this datatype to any [literal] that is indicated using child text nodes on the [RDFa statement]. For example:

Example 

<head about="">
    <h2 property="dc:title">
      E = mc<sup>2</sup>: The Most Urgent Problem of Our Time
    <h2>

</head>

would generate the expected triple:

Example 

<> dc:title
   "E = mc<sup>2</sup>: The Most Urgent Problem of Our Time"^^rdf:XMLLiteral .

Note that the value of this [XML Literal] is the exclusive canonicalization of the RDFa element's value.

clarify canonicalization

as per Elias's email, we need to clarify what this canonicalization is.

5.1.1.2. Language Tags

RDF allows [plain literal]s to have a language tag, as illustrated by the following example from [RDFTESTS-RDFMS-XMLLANG-TEST006] :

Example 

<http://example.org/node> <http://example.org/property> "chat"@fr .

In RDFa the XML language attribute -- xml:lang -- is used to add this information, whether the plain literal is designated by the content attribute, or by a datatype value of plaintext :

Example 

<meta about="http://example.org/node"
      property="ex:property" xml:lang="fr" content="chat" />

Note that the value can be inherited as defined in [XML-LANG] , so the following syntax will give the same triple as above:

Example 

<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="fr">
  <head>
    <title xml:lang="en">Example</title>
    <meta about="http://example.org/node"
      property="ex:property" content="chat" />
  </head>
  ...
</html>

5.1.2. With datatype

RDF allows [literal]s to be given a data type, as illustrated by the following example from [RDFTESTS-DATATYPES-TEST001] :

Example 

<http://example.org/foo>
  <http://example.org/bar>
  "10"^^<http://www.w3.org/2001/XMLSchema#integer> .

This can be represented in RDFa as follows:

Example 

<span about="http://example.org/foo"
      property="ex:bar" content="10" datatype="xsd:integer">ten</span>

5.1.2.1. Literal from string value of meta

If the datatype is specified, but no content attribute exists, then the typed literal's value is determined as the concatenation of all textual child elements. For example, the following RDFa:

Example 

<span about="http://example.org/foo"
      property="dc:creator" datatype="xsd:string">
  <b>M</b>ark <b>B</b>irbeck
</span>.

will yield the following triple:

Example 

<http://example.org/foo> dc:creator "Mark Birbeck"^^xsd:string .

EliasT comments

We need to explain which datatypes are allowed and emphasize "plaintext".

5.2. Blank nodes

A [blank node] is generated explicitly when an [RDFa statement] uses a bnode CURIE as its subject. A [blank node] can be generated more implicitly when an XML element without an about attribute has meta or link child elements, also without about attributes of their own. In the latter case, the [unique anonymous ID] generated to identify the [blank node] is associated with the [context statement] of the meta and link elements. This allows a number of statements to be made about the same [blank node].

For example, to establish relationships between a [blank node] and literals or URIs, one can use the implicit [blank node] construction of our earlier example, repeated here:

Example 

<blockquote>
    <link rel="dc:source" href="urn:isbn:0140449132" />
    <meta property="dc:creator" content="Fyodor Dostoevsky" />
    <p>
        Rodion Romanovitch! My dear friend! If you go on in this way
        you will go mad, I am positive! Drink, pray, if only a few drops!
    </p>
</blockquote>

This would generate the following [triple]s:

Example 

_:a dc:source <urn:isbn:0140449132> .
_:a dc:creator "Fyodor Dostoevsky" .

One could also use the more explicit declaration:

Example 

<blockquote about="[_:a]">
    <p>
        Rodion Romanovitch! My dear friend! If you go on in this way
        you will go mad, I am positive! Drink, pray, if only a few drops!
    </p>
</blockquote>
<link about="[_:a]" 
      rel="dc:source" href="urn:isbn:0140449132" />
<meta about="[_:a]"
      property="dc:creator" content="Fyodor Dostoevsky" />

To establish relationships between [blank node]s, the [unique anonymous ID] must be set explicity using a CURIE bnode as subject or object. For example, if our desired output is the following [triple]s:

Example 

_:a foaf:mbox <mailto:daniel.brickley@bristol.ac.uk> .
_:b foaf:mbox <mailto:libby.miller@bristol.ac.uk> .
_:a foaf:knows _:b .

we could use the following XHTML:

Example 

<link about="[_:a]" rel="foaf:mbox"
      href="mailto:daniel.brickley@bristol.ac.uk" />
<link about="[_:b]" rel="foaf:mbox"
      href="mailto:libby.miller@bristol.ac.uk" />
<link about="[_:a]" rel="foaf:knows"
      href="[_:b]" />

or, alternatively, if we wish to partly render the information in XHTML:

Example 

<div about="[_:a]">
   DanBri can be reached via
   <a rel="foaf:mbox" 
      href="mailto:daniel.brickley@bristol.ac.uk">
     email
   </a>.
   He knows Libby.
   <link rel="foaf:knows" href="[_:b]" />
</div>
<div about="[_:b]">
   Libby can be reached via
   <a rel="foaf:mbox"
      href="mailto:libby.miller@bristol.ac.uk">
     email
   </a>
</div>

5.3. Reification

RDFa partially supports reification.

During subject resolution (which could be triggered by object resolution for a rev attribute), the processor may traverse up the DOM tree in search of an about attribute. If a link or meta element is encountered before an about attribute is found, and if this link or meta element itself does not have an about attribute, then the subject (or, again in the case of rev , object) is resolved as the [RDFa statement] represented by this link or meta element.

For example, the following XHTML:

Example 

<div about="">
  <link rel="cc:license"
    href="http://creativecommons.org/licenses/by-nc-nd/3.0/">
     <meta property="dc:creator" content="Ben Adida" />
  </link>
  <meta property="dc:creator" content="Mark Birbeck" />
</div>

will yield the following triples:

Example 

 <> cc:license <http://creativecommons.org/licenses/by-nc-nd/3.0/> .
 <> dc:creator "Mark Birbeck."  
 _:a rdf:type rdf:Statement .
 _:a rdf:subject <> .
 _:a rdf:predicate cc:license .
 _:a rdf:object <http://creativecommons.org/licenses/by-nc-nd/3.0/> .
 _:a dc:creator "Ben Adida" .

which means that "Mark Birbeck" is the creator of the current document, that this document is licensed under a Creative Commons license, and that "Ben Adida" is the creator of that licensing statement, not of the document itself.

6. Examples

6.1. Creative Commons

One of the advantages of using the same syntax to make general statements as well as statements about a document is that in many cases a document can carry its own metadata. For example, if an XHTML document contains a navigable link to the Creative Commons license, this link can also be used to express metadata:

Example 

<div about="">
    This document is licensed under a
    <a rel="cc:license" 
       href="http://creativecommons.org/licenses/by-sa/2.0/">
        Creative Commons License
    </a>
    which, among other things, requires that you provide 
    attribution to the author,
    <a rel="dc:creator" href="http://ben.adida.net">Ben Adida</a>.
</div>

This chunk of XHTML will generate the same triples, no matter what other XHTML contains it:

Example 

<> cc:license <http://creativecommons.org/licenses/by-sa/2.0/> .
<> dc:creator <http://ben.adida.net> .

6.2. FOAF

FOAF requires the definition of at least two RDF entities: the FOAF person, and the FOAF homepage, which cannot be the same. Thus, the following XHTML can be used to represent a FOAF record:

Example 

<html xmlns:geo="http://www.w3.org/2003/01/geo/" ...>
    <head>
      <title property="dc:title">Dan's home page</title>
    </head>
    <body>
      <section id="person">
        <span about="[_:geolocation]">
          Dan is located at latitude
          <meta property="geo:lat">51.47026</meta>
          and longitude
          <meta property="geo:long">-2.59466</meta>
        </span>
        <link rel="rdf:type" href="[foaf:Person]" />
        <link rel="foaf:homepage" href="" />
        <link rel="foaf:based_near" href="[_:geolocation]" />
        <h1 property="foaf:name">Dan Brickley</h1>
      </section>
    </body>
</html>

which yields the correct FOAF triples:

Example 

<> dc:title "Dan's home page"^^rdf:XMLLiteral .
_:geolocation geo:lat "51.47026"^^rdf:XMLLiteral .
_:geolocation geo:long "-2.59466"^^rdf:XMLLiteral .
<#person> rdf:type foaf:Person .
<#person> foaf:homepage <> .
<#person> foaf:based_near _:geolocation .
<#person> foaf:name "Dan Brickley"^^rdf:XMLLiteral .

If one wants to make the foaf:Person a blank node, then the only change required is taking out the id="person" from the span element, which then yields the following triples:

Example 

<> dc:title "Dan's home page" .
_:geolocation geo:lat "51.47026" .
_:geolocation geo:long "-2.59466" .
_:span0 rdf:type foaf:Person .
_:span0 foaf:homepage <> .
_:span0 foaf:based_near _:geolocation .
_:span0 foaf:name "Dan Brickley" .

7. Rules

(not updated yet.)

A. References

DC
Dublin Core Metadata Initiative (DCMI) (See http://dublincore.org/ .)
FOAF-PROJECT
The FOAF Project (See http://www.foaf-project.org .)
HTML
HTML 4.01 Specification (See http://www.w3.org/TR/html401/ .)
IRI
" Internationalized Resource Identifiers (IRI) ", RFC 3987, M.Duerst, M. Suignard January 2005.
Available at: http://www.ietf.org/rfc/rfc3987.txt
N-TRIPLES
RDF Test Cases, N-Triples (See http://www.w3.org/TR/rdf-testcases/#ntriples .)
N3-PRIMER
N3 Primer (See http://www.w3.org/2000/10/swap/Primer .)
RDF-CONCEPTS
Resource Description Framework (RDF): Concepts and Abstract Syntax (See http://www.w3.org/TR/rdf-concepts/ .)
RDF-SYNTAX
RDF/XML Syntax and Grammar (See http://www.w3.org/TR/rdf-syntax-grammar/ .)
RDFTESTS-DATATYPES-TEST001
datatypes/test001.nt (See http://www.w3.org/2000/10/rdf-tests/rdfcore/datatypes/test001.nt .)
RDFTESTS-RDFMS-XMLLANG-TEST006
rdfms-xmllang/test006.nt (See http://www.w3.org/2000/10/rdf-tests/rdfcore/rdfms-xmllang/test006.nt .)
RELAXNG
RELAX NG Home Page (See http://www.relaxng.org/ .)
SWD-WG
Semantic Web Best Deployment Working Group (See http://www.w3.org/2006/07/SWD/ .)
XHTML
XHTML 1.0 (See http://www.w3.org/TR/xhtml1 .)
XHTML-2.0-LINKTYPES
XHTML 2.0 Link Types (See http://www.w3.org/MarkUp/Group/2003/WD-xhtml2-20031029/abstraction.html#dt_LinkTypes .)
XML-BASE
XML Base (See http://www.w3.org/TR/xmlbase/ .)
XML-LANG
Extensible Markup Language (XML) 1.0 (Third Edition), Language Identification (See http://www.w3.org/TR/REC-xml/#sec-lang-tag .)
XPOINTER-FRAMEWORK
XPointer Framework (See http://www.w3.org/TR/xptr-framework/ .)
RDFHTML
RDF-in-HTML Task Force (See http://w3.org/2001/sw/BestPractices/HTML/ .)
SWBPD-WG
Semantic Web Best Practices and Deployment Working Group (See http://w3.org/2001/sw/BestPractices/ .)
HTML-WG
HTML Working Group (See http://w3.org/MarkUp/Group/ .)
CURIE
CURIEs (See http://w3.org/2001/sw/BestPractices/HTML/2005-10-27-CURIE .)

B. Change History

2007-04-06: fixed some of the language to talk about "structure" rather than metadata. Added note regarding space-separated values in predicate-denoting attributes. [BenAdida]

2006-01-16: made the use of CURIE type for rel , rev , property consistent across document (particularly section 2.4 was erroneous). [BenAdida]

C. Acknowledgments

This section is informative.

At the time of publication, the participants in the W3C XHTML 2 Working Group were: