RDFa Core 1.1 (original) (raw)

Syntax and processing rules for embedding RDF through attributes

W3C Working Draft 31 March 2011

This version:

http://www.w3.org/TR/2011/WD-rdfa-core-20110331/

Latest published version:

http://www.w3.org/TR/rdfa-core/

Latest editor's draft:

http://www.w3.org/2010/02/rdfa/drafts#rdfa-core

Previous version:

http://www.w3.org/TR/2010/WD-rdfa-core-20101026/

Latest recommendation:

http://www.w3.org/TR/rdfa-syntax/

Editors:

Ben Adida, Creative Commons ben@adida.net

Mark Birbeck, webBackplane mark.birbeck@webBackplane.com

Shane McCarron, Applied Testing and Technology, Inc. shane@aptest.com

Ivan Herman, W3C ivan@w3.org

This document is also available in these non-normative formats: Diff from previous Working Draft, PostScript version, and PDF version.

Copyright © 2007-2011 W3C® (MIT, ERCIM, Keio), All Rights Reserved. W3C liability, trademark and document use rules apply.

Abstract

The current Web is primarily made up of an enormous number of documents that have been created using HTML. These documents contain significant amounts of structured data, which is largely unavailable to tools and applications. 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, and allowing browsing applications to improve the user experience: 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, location and topic can be published as easily as the original photo itself, enabling structured search and sharing.

RDFa Core is a specification for attributes to express structured data in any markup language. The embedded data already available in the markup language (e.g., XHTML) can often be reused by the RDFa markup, so that publishers don't need to repeat significant data in the document content. The underlying abstract representation is RDF [RDF-PRIMER], 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 of the same goals with microformats [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 vocabulary being used.

This document is a detailed syntax specification for RDFa, aimed at:

• those looking to create an RDFa Processor, and who therefore need a detailed description of the parsing rules;
• those looking to integrate RDFa into a new markup language;
• those looking to recommend the use of RDFa within their organization, and who would like to create some guidelines for their users;
• anyone familiar with RDF, and who wants to understand more about what is happening 'under the hood', when an RDFa Processor runs.

For those looking for an introduction to the use of RDFa and some real-world examples, please consult the RDFa Primer.

How to Read this Document

First, if you are not familiar with either RDFa or RDF, and simply want to add RDFa to your documents, then you may find the RDFa Primer [RDFA-PRIMER] to be a better introduction.

If you are already familiar with RDFa, and you want to examine the processing rules — perhaps to create an RDFa Processor — then you'll find the Processing Model section of most interest. It contains an overview of each of the processing steps, followed by more detailed sections, one for each rule.

If you are not familiar with RDFa, but you are familiar with RDF, then you might find reading the Syntax Overview useful, before looking at the Processing Model since it gives a range of examples of markup that use RDFa. Seeing some examples first should make reading the processing rules easier.

If you are not familiar with RDF, then you might want to take a look at the section on RDF Terminologybefore trying to do too much with RDFa. Although RDFa is designed to be easy to author — and authors don't need to understand RDF to use it — anyone writing applications that consume RDFa will need to understand RDF. There is a lot of material about RDF on the web, and a growing range of tools that support RDFa, this document only contains enough background on RDF to make the goals of RDFa more clear.

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 version reflects changes made as a result of comments received during the first Last Call period. That document was widely reviewed and the Working Group has made a number of improvements and clarifications as a result. The Working Group believes that all comments received during the last review have been addressed.

This is a revision of RDFa Syntax 1.0 [RDFA-SYNTAX]. Once development is complete, if accepted by the W3C membership, this document will supersede the previous Recommendation. There are a number of substantive differences between this version and its predecessor, including:

1. The removal of the specific rules for XHTML - these are now defined in XHTML+RDFa [XHTML-RDFA]
2. An expansion of the datatypes of some RDFa attributes so that they can contain Terms, CURIES, or Absolute URIs.
3. The ability to reference RDFa Profiles; these are used to ease authoring by creating collections of terms, prefix definitions, and/or default vocabulary declarations.
4. Host languages are permitted to define collections of default terms, default prefix mappings, and a default vocabulary mapping via a default RDFa Profile.
5. The ability to define a default vocabulary to use for Terms that are undefined.
6. Terms are required to be compared in a case-insensitive manner.

A sample test harness is available. This set of tests is not intended to be exhaustive. Users may find the tests to be useful examples of RDFa usage.

This document was published by the RDFa Working Group as a Last Call Working Draft. This document is intended to become a W3C Recommendation. If you wish to make comments regarding this document, please send them to public-rdfa-wg@w3.org (subscribe, archives). The Last Call period ends 21 April 2011. All feedback is welcome.

Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

This is a Last Call Working Draft and thus the Working Group has determined that this document has satisfied the relevant technical requirements and is sufficiently stable to advance through the Technical Recommendation process.

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

Table of Contents

• 1. Motivation
• 2. Syntax Overview
  • 2.1 The RDFa Attributes
  • 2.2 Examples
• 3. RDF Terminology
  • 3.1 Statements
  • 3.2 Triples
  • 3.3 URI references
  • 3.4 Plain literals
  • 3.5 Typed literals
  • 3.6 Turtle
  • 3.7 Graphs
  • 3.8 Compact URI Expressions
  • 3.9 Markup Fragments and RDFa
  • 3.10 A description of RDFa in RDF terms
• 4. Conformance
  • 4.1 RDFa Processor Conformance
  • 4.2 RDFa Host Language Conformance
  • 4.3 XML+RDFa Document Conformance
• 5. Attributes and Syntax
  • 5.1 White space within attribute values
• 6. CURIE Syntax Definition
  • 6.1 Why CURIEs and not QNames?
• 7. Processing Model
  • 7.1 Overview
  • 7.2 Evaluation Context
  • 7.3 Chaining
  • 7.4 CURIE and URI Processing
    * 7.4.1 Scoping of Prefix Mappings
    * 7.4.2 General Use of CURIEs in Attributes
    * 7.4.3 General Use of Terms in Attributes
    * 7.4.4 Use of CURIEs in Specific Attributes
    * 7.4.5 Referencing Blank Nodes
  • 7.5 Sequence
  • 7.6 Processor Status
    * 7.6.1 Accessing the Processor Graph
    * 7.6.2 Processor Graph Terms
• 8. RDFa Processing in detail
  • 8.1 Changing the evaluation context
    * 8.1.1 Setting the current subject
    * 8.1.1.1 The current document
    * 8.1.1.2 Using @about
    * 8.1.1.3 Using @src
    * 8.1.1.4 Creating a new item with @typeof
    * 8.1.1.5 Determining the subject with neither @about nor @typeof
    * 8.1.1.5.1 Inheriting subject from @resource
    * 8.1.1.5.2 Inheriting an anonymous subject
  • 8.2 Completing incomplete triples'
  • 8.3 Object resolution
    * 8.3.1 Literal object resolution
    * 8.3.1.1 Plain Literals
    * 8.3.1.1.1 Language Tags
    * 8.3.1.2 Typed literals
    * 8.3.1.3 XML Literals
    * 8.3.2 URI object resolution
    * 8.3.2.1 Using @resource to set the object
    * 8.3.2.2 Using @href
    * 8.3.2.3 Incomplete triples
• 9. RDFa Profiles
• A. CURIE Datatypes
  • A.1 XML Schema Definition
  • A.2 XML DTD Definition
• B. The RDFa Vocabulary
  • B.1 Term and Prefix Assignments
  • B.2 Processor Graph Reporting
• C. Changes
  • C.1 Major differences with RDFa Syntax 1.0
  • C.2 Major changes during development of version 1.1
• D. Acknowledgments
• E. References
  • E.1 Normative references
  • E.2 Informative references

1. Motivation

This section is non-normative.

RDF/XML [RDF-SYNTAX] provides sufficient flexibility to represent all of the abstract concepts in RDF [RDF-CONCEPTS]. 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 [RELAXNG-SCHEMA] 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 in the text—perhaps as a byline on a news site—should not need to repeat this name for the RDF expression of the same concept: it should be possible to supplement the existing markup in such a way that it can also be interpreted as RDF.

Another reason for aligning the rendered data with the structured data is that it is highly beneficial to express the web data's structure 'in context'; as users often want to transfer structured data from one application to another, sometimes to or from a non-web-based application, the user experience can be enhanced. For example, information about specific rendered data could be presented to the user via 'right-clicks' on an item of interest.

In the past, many attributes were 'hard-wired' directly into the markup language to represent specific concepts. For example, in XHTML 1.1 [XHTML11] and HTML [HTML401] there is @cite; the attribute allows an author to add information to a document which is used to indicate the origin of a quote.

However, these 'hard-wired' attributes make it difficult to define a generic process for extracting metadata from any document since an RDFa Processor would need to know about each of the special attributes. 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. In most cases the values of those properties are the information that is already in an author's document.

RDFa alleviates the pressure on markup language designers to anticipate all the structural requirements users of their language might have, by outlining a new syntax for RDF that relies only on attributes. By adhering to the concepts and rules in this specification, language designers can import RDFa into their environment with a minimum of hassle and be confident that semantic data will be extractable from their documents by conforming processors.

2. Syntax Overview

This section is non-normative.

The following examples are intended to help readers who are not familiar with RDFa to quickly get a sense of how it works. For a more thorough introduction, please read the RDFa Primer [RDFA-PRIMER].

In RDF, it is common for people to shorten vocabulary terms via abbreviated URIs that use a 'prefix' and a 'reference'. This mechanism is explained in detail in the section titled Compact URI Expressions. The examples throughout this document assume that the following vocabulary prefixes have been defined:

In some of the examples below we have used URIs with fragment identifiers that are local to the document containing the RDFa fragment identifiers shown (e.g., 'about="#me"'). This idiom, which is also used in RDF/XML [RDF-SYNTAX-GRAMMAR] and other RDF serializations, gives a simple way to 'mint' new URIs for entities described by RDFa and therefore contributes considerably to the expressive power of RDFa. However, the media type registrations that govern the meaning of fragment identifiers (see section 3.5 of the URI specification [RFC3986], [RFC3023], and [RFC2854]) have not yet caught up with this practice. For more information about fragment identifier semantics, see [WEBARCH] section 3.2.1.

2.1 The RDFa Attributes

RDFa makes use of a number of commonly found attributes, as well as providing a few new ones. Attributes that already exist in widely deployed languages (e.g., HTML) have the same meaning they always did, although their syntax has been slightly modified in some cases. For example, in (X)HTML there is no clear way to add new @rel values; RDFa sets out to explicitly solve this problem, and does so by allowing URIs as values. It also introduces the concepts of terms and 'compact URI expressions' — referred to as CURIEs in this document — which allow a full URI value to be expressed succinctly. For a complete list of RDFa attribute names and syntax, see Attributes and Syntax.

2.2 Examples

In HTML, authors can include metadata and relationships concerning the current document by using the meta and link elements. For example, the author of the page along with the pages preceding and following the current page can be expressed using the link and meta elements:

RDFa makes use of this concept, enhancing it with the ability to make use of other vocabularies by using full URIs:

Because using full URIs like those above can be cumbersome, RDFa also permits the use of compact URI expressions so an author can use a shorthand to reference terms in multiple vocabularies:

RDFa supports the use of @rel and @rev on any element. This is even more useful with the addition of support for different vocabularies:

This document is licensed under the Creative Commons By-NC-ND License .

Not only can URIs in the document be re-used to provide metadata, but so can inline text when used with @property:

I'm holding one last summer Barbecue , on September 16th at 4pm.

If some displayed text is different to the actual 'value' it represents, a more precise value can be added, which can optionally include @datatype:

I'm holding one last summer Barbecue , on September 16th at 4pm .

In many cases a block of markup will contain a number of properties that relate to the same item; it's possible with RDFa to indicate the type of that item using@typeof:

I'm holding one last summer Barbecue , on September 16th at 4pm .

RDFa allows the document to contain metadata information about other documents and resources:

When dealing with small amounts of markup, it is sometimes easier to use full URIs, rather than CURIEs. The previous example can also be written as follows:

A simple way of defining a portion of a document using terms from a specific vocabulary is to use @vocab to define a default vocabulary URI. For example, to use FOAF terms:

The example above will produce the following triples, expressed here in Turtle syntax:

@prefix foaf: http://xmlns.com/foaf/0.1/ . <#me> foaf:name "John Doe" ; foaf:homepage http://example.org/blog/ .

RDFa also permits external definition of collections of prefixes. The following (mythical) example RDFa Profile document, with a URI of http://www.example.org/vocab-rdf-dc.html, defines some standard RDF prefixes as well as the FOAF and Dublin Core vocabulary prefixes in RDFa.

This is an example to defining the standard RDF and Dublin Core prefixes

<p typeof="rdfa:PrefixMapping">
   The "<span property="rdfa:prefix">rdf</span>" prefix can
   be used for the URI: 
   "<span property="rdfa:uri">http://www.w3.org/1999/02/22-rdf-syntax-ns#</span>".</p>

<p typeof="rdfa:PrefixMapping">
   The "<span property="rdfa:prefix">rdfs</span>" prefix can 
   be used for the URI: 
   "<span property="rdfa:uri">http://www.w3.org/2000/01/rdf-schema#</span>".</p>

<p typeof="rdfa:PrefixMapping">
   The "<span property="rdfa:prefix">dcterms</span>" prefix can
   be used for the URI: 
   "<span property="rdfa:uri">http://purl.org/dc/terms/</span>".</p>

<p typeof="rdfa:PrefixMapping">
   The "<span property="rdfa:prefix">foaf</span>" prefix can
   be used for the URI: 
   "<span property="rdfa:uri">http://xmlns.com/foaf/0.1/</span>".</p>

The same content in Turtle is:

@prefix rdfa: <rdfa: http://www.w3.org/ns/rdfa#> . @prefix dcterms: http://purl.org/dc/terms/ . [] a rdfa:PrefixMapping ; rdfa:prefix "rdf" ; rdfa:uri "http://www.w3.org/1999/02/22-rdf-syntax-ns#" . [] a rdfa:PrefixMapping rdfa:prefix "rdfs" ; rdfa:uri "http://www.w3.org/2000/01/rdf-schema#" . [] a rdfa:PrefixMapping ; rdfa:prefix "dcterms" ; rdfa:uri "http://purl.org/dc/terms/" . [] a rdfa:PrefixMapping ; rdfa:prefix "foaf" ; rdfa:uri "http://xmlns.com/foaf/0.1/" .

Using @profile, the following RDFa snippet:

title of the document and this is a longer comment on the same document

would yield the following triples:

@prefix rdfs: http://www.w3.org/2000/01/rdf-schema# . @prefix dcterms: http://purl.org/dc/terms/ . http://www.example.org/doc dcterms:title "title of the document" ; rdfs:comment "and this is a longer comment on the same document" .

It is also possible to define terms. Given the following RDFa Profile document at http://www.example.org/vocab-foaf-terms.html:

This is an example RDFa vocabulary that makes it easier to use the foaf:name and foaf:homepage terms.

The "name" term can be used for the URI: "http://xmlns.com/foaf/0.1/name".

The "homepage" term can be used for the URI: "http://xmlns.com/foaf/0.1/homepage".

and the following HTML markup:

the following triples will be generated:

@prefix foaf: http://xmlns.com/foaf/0.1/ . <#me> foaf:name "John Doe" ; foaf:homepage http://example.org/blog/ .

3. RDF Terminology

This section is non-normative.

The previous section gave examples of typical markup in order to illustrate the structure of RDFa markup. RDFa is short for "RDF in Attributes". In order to author RDFa you do not need to understand RDF, although it would certainly help. However, if you are building a system that consumes the RDF output of a language that supports RDFa you will almost certainly need to understand RDF. This section introduces the basic concepts and terminology of RDF. For a more thorough explanation of RDF, please refer to the RDF Concepts document [RDF-CONCEPTS] and the RDF Syntax Document [RDF-SYNTAX].

3.1 Statements

The structured data that RDFa provides access to is a collection of statements. A statement is a basic unit of information that has been constructed in a specific format to make it easier to process. In turn, by breaking large sets of information down into a collection of statements, even very complex metadata can be processed using simple rules.

To illustrate, suppose we have the following set of facts:

Albert was born on March 14, 1879, in the German Empire. There is a picture of him at the web address, http://en.wikipedia.org/wiki/Image:Albert_Einstein_Head.jpg.

This would be quite difficult for a machine to interpret, and it is certainly not in a format that could be passed from one data application to another. However, if we convert the information to a set of statements it begins to be more manageable. The same information could therefore be represented by the following shorter 'statements':

Albert was born on March 14, 1879. Albert was born in the German Empire. Albert has a picture at http://en.wikipedia.org/wiki/Image:Albert_Einstein_Head.jpg.

3.2 Triples

To make this information machine-processable, RDF defines a structure for these statements. A statement is formally called a triple, meaning that it is made up of three components. The first is the subject of the triple, and is what we are making our statements about. In all of these examples the subject is 'Albert'.

The second part of a triple is the property of the subject that we want to define. In the examples here, the properties would be 'was born on', 'was born in', and 'has a picture at'. These are more usually called predicates in RDF.

The final part of a triple is called the object. In the examples here the three objects have the values 'March 14, 1879', 'the German Empire', and 'http://en.wikipedia.org/wiki/Image:Albert\_Einstein\_Head.jpg'.

3.3 URI references

Breaking complex information into manageable units helps us be specific about our data, but there is still some ambiguity. For example, which 'Albert' are we talking about? If another system has more facts about 'Albert', how could we know whether they are about the same person, and so add them to the list of things we know about that person? If we wanted to find people born in the German Empire, how could we know that the predicate 'was born in' has the same purpose as the predicate 'birthplace' that might exist in some other system? RDF solves this problem by replacing our vague terms with URI references.

URIs are most commonly used to identify web pages, but RDF makes use of them as a way to provide unique identifiers for concepts. For example, we could identify the subject of all of our statements (the first part of each triple) by using the DBPedia [http://dbpedia.org] URI for Albert Einstein, instead of the ambiguous string 'Albert':

http://dbpedia.org/resource/Albert_Einstein has the name Albert Einstein. http://dbpedia.org/resource/Albert_Einstein was born on March 14, 1879. http://dbpedia.org/resource/Albert_Einstein was born in the German Empire. http://dbpedia.org/resource/Albert_Einstein has a picture at http://en.wikipedia.org/wiki/Image:Albert_Einstein_Head.jpg.

URI references are also used to uniquely identify the objects in metadata statements (the third part of each triple). The picture of Einstein is already a URI, but we could also use a URI to uniquely identify the country 'German Empire'. At the same time we'll indicate that the name and date of birth really are literals (and not URIs), by putting quotes around them:

http://dbpedia.org/resource/Albert_Einstein has the name "Albert Einstein". http://dbpedia.org/resource/Albert_Einstein was born on "March 14, 1879". http://dbpedia.org/resource/Albert_Einstein was born in http://dbpedia.org/resource/German_Empire. http://dbpedia.org/resource/Albert_Einstein has a picture at http://en.wikipedia.org/wiki/Image:Albert_Einstein_Head.jpg.

URI references are also used to ensure that predicates are unambiguous; now we can be sure that 'birthplace', 'place of birth', 'Lieu de naissance' and so on, all mean the same thing:

http://dbpedia.org/resource/Albert_Einstein http://xmlns.com/foaf/0.1/name "Albert Einstein". http://dbpedia.org/resource/Albert_Einstein http://dbpedia.org/property/dateOfBirth "March 14, 1879". http://dbpedia.org/resource/Albert_Einstein http://dbpedia.org/property/birthPlace http://dbpedia.org/resource/German_Empire. http://dbpedia.org/resource/Albert_Einstein http://xmlns.com/foaf/0.1/depiction http://en.wikipedia.org/wiki/Image:Albert_Einstein_Head.jpg.

3.4 Plain literals

Although URI resources are always used for subjects and predicates, the object part of a triple can be either a URI or a_literal_. In the example triples, Einstein's name is represented by a plain literal, which means that it is a basic string with no type or language information:

http://dbpedia.org/resource/Albert_Einstein http://xmlns.com/foaf/0.1/name "Albert Einstein".

3.5 Typed literals

Some literals, such as dates and numbers, have very specific meanings, so RDF provides a mechanism for indicating the type of a literal. A typed literal is indicated by attaching a URI to the end of a plain literal, and this URI indicates the literal's datatype. This URI is usually based on datatypes defined in the XML Schema Datatypes specification [XMLSCHEMA-2]. The following syntax would be used to unambiguously express Einstein's date of birth as a literal of type http://www.w3.org/2001/XMLSchema#date:

http://dbpedia.org/resource/Albert_Einstein http://dbpedia.org/property/dateOfBirth "1879-03-14"^^http://www.w3.org/2001/XMLSchema#date.

3.6 Turtle

RDF itself does not have one set way to express triples, since the key ideas of RDF are the triple and the use of URIs, and not any particular syntax. However, there are a number of mechanisms for expressing triples, such as RDF/XML [RDF-SYNTAX-GRAMMAR], Turtle [TURTLE], and of course RDFa. Many discussions of RDF make use of the Turtle syntax to explain their ideas, since it is quite compact. The examples we have just seen are already using this syntax, and we'll continue to use it throughout this document when we need to talk about the RDF that could be generated from some RDFa. Turtle allows long URIs to be abbreviated by using a URI mapping, which can be used to express a compact URI expression as follows:

@prefix dbp: http://dbpedia.org/property/ . @prefix foaf: http://xmlns.com/foaf/0.1/ . http://dbpedia.org/resource/Albert_Einstein foaf:name "Albert Einstein" . http://dbpedia.org/resource/Albert_Einstein dbp:birthPlace http://dbpedia.org/resource/German_Empire .

Here 'dbp:' has been mapped to the URI for DBPedia and 'foaf:' has been mapped to the URI for the 'Friend of a Friend' taxonomy.

Any URI in Turtle could be abbreviated in this way. This means that we could also have used the same technique to abbreviate the identifier for Einstein, as well as the datatype indicator:

@prefix dbp: http://dbpedia.org/property/ . @prefix dbr: http://dbpedia.org/resource/ . @prefix foaf: http://xmlns.com/foaf/0.1/ . @prefix xsd: http://www.w3.org/2001/XMLSchema# . dbr:Albert_Einstein dbp:dateOfBirth "1879-03-14"^^xsd:date . dbr:Albert_Einstein foaf:depiction http://en.wikipedia.org/wiki/Image:Albert_Einstein_Head.jpg .

When writing examples, you will often see the following URI in the Turtle representation:

<>

This indicates the 'current document', i.e., the document being processed. In reality there would always be a full URI based on the document's location, but this abbreviation serves to make examples more compact. Note in particular that the whole technique of abbreviation is merely a way to make examples more compact, and the actual triples generated would always use the full URIs.

3.7 Graphs

A collection of triples is called a graph. All of the triples that are defined by this specification are contained in the default graph by an RDFa Processor. For more information on graphs and other RDF concepts, see [RDF-CONCEPTS].

3.8 Compact URI Expressions

In order to allow for the compact expression of RDF statements, RDFa allows the contraction of most URI references into a form called a 'compact URI expression', or CURIE. A detailed discussion of this mechanism is in the section CURIE and URI Processing.

Note that CURIEs are only used in the markup and Turtle examples, and will never appear in the generated triples, which are defined by RDF to use URI references.

Full details on how CURIEs are processed are in the section titled CURIE Processing.

3.9 Markup Fragments and RDFa

A growing use of embedded metadata is to take fragments of markup and move them from one document to another. This may happen through the use of tools, such as drag-and-drop in a browser, or through snippets of code provided to authors for inclusion in their documents. (A good example of the latter is the licensing fragment provided by Creative Commons.)

However, those involved in creating fragments (either by building tools, or authoring snippets), should be aware that this specification does not say how fragments are processed. Specifically, the processing of a fragment 'outside' of a complete document is undefined because RDFa processing is largely about context. Future versions of this or related specifications may do more to define this behavior.

Developers of tools that process fragments, or authors of fragments for manual inclusion, should also bear in mind what will happen to their fragment once it is included in a complete document. They should carefully consider the amount of 'context' information that will be needed in order to ensure a correct interpretation of their fragment.

3.10 A description of RDFa in RDF terms

The following is a brief description of RDFa in terms of the RDF terminology introduced here. It may be useful to readers with an RDF background:

The aim of RDFa is to allow a single RDF graph to be carried in various types of document markup. An RDF graph comprises nodes linked by relationships. The basic unit of an RDF graph is a triple, in which a subject node is linked to an object node via a predicate. Thesubject node is always either a URI reference or a blank node (or bnode), the predicate is always a URI reference, and the object of a statement can be a URI reference, aliteral, or a bnode.

In RDFa, a subject URI reference is generally indicated using @about or @src, and predicates are represented using one of @property,@rel, or@rev. Objects which are URI references are represented using @resource, or @href, whilst objects that are literals are represented either with @content or the content of the element in question (with an optional datatype expressed using@datatype, and an optional language expressed using a Host Language-defined mechanism such as xml:lang).

4. Conformance

As well as sections marked as non-normative, all authoring guidelines, diagrams, examples, and notes in this specification are non-normative. Everything else in this specification is normative.

The key words must, must not, required, should, should not, recommended, may, and optional in this specification are to be interpreted as described in [RFC2119].

4.1 RDFa Processor Conformance

A conforming RDFa Processor must make available to a consuming application a single RDF graph containing all possible triples generated by using the rules in the Processing Model section. This specification uses the termdefault graph to mean all of the triples asserted by a document according to the Processing Model section. The term processor graph is used to denote the collection of all informational, warning, and error triples that may be generated by the RDFa Processor to report its status. The default graphand the processor graph are separate graphs and must not be stored in the same graph by the RDFa Processor.

A conforming RDFa Processor may make available additional triples that have been generated using rules not described here, but these triples must not be made available in the default graph. (Whether these additional triples are made available in one or more additional RDF graphs is implementation-specific, and therefore not defined here.)

A conforming RDFa Processor_must_ preserve white space in both plain literals and XML literals. However, it may be the case that the architecture in which a processor operates has made changes to the white space in a document before that document ever reaches the RDFa Processor (e.g., [XMLSCHEMA-1] processors are permitted to 'normalize' white space in attribute values - see section 3.1.4). To ensure maximum consistency between processing environments, authors should remove any unnecessary white space in their plain and XML Literal content.

A conforming RDFa Processor must examine the media type of a document it is processing to determine the document's Host Language. If the RDFa Processor is unable to determine the media type, or does not support the media type, the RDFa Processor must process the document as if it were media type application/xml. See XML+RDFa Document Conformance.

A conforming RDFa Processor may use additional mechanisms (e.g., the DOCTYPE, a file extension, the root element) to attempt to determine the Host Language if the media type is unavailable. These mechanisms are unspecified.

4.2 RDFa Host Language Conformance

Host Languages that incorporate RDFa must adhere to the following:

• All of the facilities required in this specification must be included in the Host Language.
• The attributes defined in this specification must be included in the content model of the Host Language.
• If the Host Language uses XML Namespaces [XML-NAMES], the attributes in this specification should be defined in 'no namespace'. (e.g., when the attributes are used on elements in the Host Language's namespace, they can be used with no qualifying prefix: <myml:myElement property="next">).
• If the Host Language has its own definition for any attribute defined in this specification, that definition_must_ be such that the processing required by this specification remains possible when the attribute is used in a way consistent with the requirements herein.
• The Host Language may require the automatic inclusion of one or more default RDFa Profiles. If it does, the RDFa Profile triples that establish term or URI mappings_must not_ change without changing the associated profile URI. RDFa Processors _may_embed, cache, or retrieve the RDFa Profile triples associated with that profile.
  Maintainers of Host Languages are required to change the URI of a default profile if items are removed from the default profile document. The URI change is required to accommodate RDFa Processors that statically embed the terms defined in the profile. The working group expects default RDFa Profiles to change very rarely.

4.3 XML+RDFa Document Conformance

This specification does not define a stand-alone document type. The attributes herein are intended to be integrated into other host languages (e.g., HTML+RDFa or XHTML+RDFa). However, this specification does define processing rules for generic XML documents - that is, those documents delivered as media types text/xml orapplication/xml. Such documents must meet all of the following criteria:

1. The document must be well-formed as defined in [XML10-4e].
2. The document must use the attributes defined in this specification in 'no namespace'.

When an RDFa Processor processes an XML+RDFa document, it does so in the following context:

1. The default vocabulary URI is undefined.
2. The default collection of terms is defined via an RDFa Profile document at http://www.w3.org/profile/rdfa-1.1.
3. The base can be set using the @xml:baseattribute as defined in [XML10-4e].
4. The current language can be set using @xml:lang attribute.

5. Attributes and Syntax

This specification defines a number of attributes and the way in which the values of those attributes are to be interpreted when generating RDF triples. This section defines the attributes and the syntax of their values.

about

a SafeCURIEorCURIEorURI, used for stating what the data is about (a 'subject' in RDF terminology);

content

a CDATA string, for supplying machine-readable content for a literal (a 'plain literal object', in RDF terminology);

datatype

a TERMorCURIEorAbsURI representing a datatype, to express the datatype of a literal;

href

a traditionally navigable URI for expressing the partner resource of a relationship (a 'resource object', in RDF terminology);

prefix

a white space separated list of prefix-name URI pairs of the form

NCName ':' ' '+ xs:anyURI

profile

a white space separated list of one or more URIs that indicate a profile of terms, prefix mappings, and/or default vocabulary declarations. See RDFa Profiles;

property

a white space separated list ofTERMorCURIEorAbsURIs, used for expressing relationships between a subject and some literal text (also a 'predicate');

rel

a white space separated list of TERMorCURIEorAbsURIs, used for expressing relationships between two resources ('predicates' in RDF terminology);

resource

a SafeCURIEorCURIEorURI for expressing the partner resource of a relationship that is not intended to be navigable (e.g., a 'clickable' link) (also an 'object');

rev

a white space separated list ofTERMorCURIEorAbsURIs, used for expressing reverse relationships between two resources (also 'predicates');

src

a URI for expressing the partner resource of a relationship when the resource is embedded (also a 'resource object');

typeof

a white space separated list ofTERMorCURIEorAbsURIs that indicate the RDF type(s) to associate with a subject;

vocab

A URI that defines the mapping to use when a TERM is referenced in an attribute value. SeeGeneral Use of Terms in Attributes.

5.1 White space within attribute values

Many attributes accept a white space separated list of tokens. This specification defines white space as:

whitespace ::= (#x20 | #x9 | #xD | #xA)+

When attributes accept a white space separated list of tokens, an RDFa Processor must ignore any leading or trailing white space.

This definition is consistent with the definition found in [XML10].

6. CURIE Syntax Definition

The key component of RDF is the URI, but these are usually long and unwieldy. RDFa therefore supports a mechanism by which URIs can be abbreviated, called 'compact URI expressions' or simply, CURIEs.

When expanded, the resulting URI must be a syntactically valid URI [RFC3987]. For a more detailed explanation seeCURIE and URI Processing. The _lexical space_of a CURIE is as defined in curie below. The value space is the set of URIs.

A CURIE is comprised of two components, a prefix and a_reference_. The prefix is separated from the reference by a colon (:). In general use it is possible to omit the prefix, and so create a CURIE that makes use of the 'default prefix' mapping; in RDFa the 'default prefix' mapping is http://www.w3.org/1999/xhtml/vocab#. It's also possible to omit both the prefix and the colon, and so create a CURIE that contains just a reference which makes use of the 'no prefix' mapping. This specification does not define a 'no prefix' mapping. RDFa Host Languages must not define a 'no prefix' mapping.

The RDFa 'default prefix' should not be confused with the 'default namespace' as defined in [XML-NAMES]. An RDFa Processor must not treat an XML-NAMES 'default namespace' declaration as if it were setting the 'default prefix'.

The general syntax of a CURIE can be summarized as follows:

prefix ::= NCName

reference ::= irelative-ref (as defined in [RFC3987])

curie ::= [ [ prefix ] ':' ] reference

safe_curie ::= '[' [ [ prefix ] ':' ] reference ']'

The production safe_curie is not required, even in situations where an attribute value is permitted to be a CURIE or a URI: A URI that uses a scheme that is not an in-scope mapping cannot be confused with a CURIE. The concept of a safe_curie is retained for backward compatibility.

In normal evaluation of CURIEs the following context information would need to be provided:

• a set of mappings from prefixes to URIs;
• a mapping to use with the default prefix (for example, :p);
• a mapping to use when there is no prefix (for example, p);
• a mapping to use with the '_' prefix, which is used to generate unique identifiers (for example, _:p).

In RDFa these values are defined as follows:

• the set of mappings from prefixes to URIs is provided by the current in-scope prefix declarations of thecurrent element during parsing;
• the mapping to use with the default prefix is the current default prefix mapping;
• the mapping to use when there is no prefix is not defined;
• the mapping to use with the '_' prefix, is not explicitly stated, but since it is used to generate bnodes, its implementation needs to be compatible with the RDF definition and rules in Referencing Blank Nodes. A document should not define a mapping for the '_' prefix. A Conforming RDFa Processor must ignore any definition of a mapping for the '_' prefix.

A CURIE is a representation of a full URI. The rules for determining that URI are:

• If a CURIE consists of an empty prefix and a reference, the URI is obtained by taking the current default prefix mapping and concatenating it with the reference. If there is no current default prefix mapping, then this is not a valid CURIE and must be ignored.
• Otherwise, if a CURIE consists of a non-empty prefix and reference, and if there is an in-scope mapping for prefix (when compared case-insensitively), then the URI is created by using that mapping, and concatenating it with thereference.
• Finally, if there is no in-scope mapping forprefix, then the value is not a CURIE.

See General Use of Terms in Attributes for the way items with no colon can be interpreted in some datatypes by RDFa Processors.

6.1 Why CURIEs and not QNames?

This section is non-normative.

In many cases, language designers have attempted to use QNames for an extension mechanism [XMLSCHEMA-2]. QNames do permit independent management of the name collection, and can map the names to a resource. Unfortunately, QNames are unsuitable in most cases because 1) the use of QName as identifiers in attribute values and element content is problematic as discussed in [QNAMES] and 2) the syntax of QNames is overly restrictive and does not allow all possible URIs to be expressed.

A specific example of the problem this causes comes from attempting to define the name collection for books. In a QName, the part after the colon must be a valid element name, making an example such as the following invalid: isbn:0321154991

This is not a valid QName simply because "0321154991" is not a valid element name. Yet, in the example given, we don't really want to define a valid element name anyway. The whole reason for using a QName was to reference an item in a private scope - that of ISBNs. Moreover, in this example, we want the names within that scope to map to a URI that will reveal the meaning of that ISBN. As you can see, the definition of QNames and this (relatively common) use case are in conflict with one another.

This specification addresses the problem by defining CURIEs. Syntactically, CURIEs are a superset of QNames.

Note that this specification is targeted at language designers, not document authors. Any language designer considering the use of QNames as a way to represent URIs or unique tokens should consider instead using CURIEs:

• CURIEs are designed from the ground up to be used in attribute values. QNames are designed for unambiguously naming elements and attributes.
• CURIEs expand to URIs, and any URI can be represented by such a mapping. QNames are treated as value pairs, but even if those pairs are combined into a string, only a subset of URIs can be represented.
• CURIEs can be used in non-XML grammars, and can even be used in XML languages that do not support XML Namespaces. QNames are limited to XML Namespace-aware XML Applications.

7. Processing Model

This section looks at a generic set of processing rules for creating a set of triples that represent the structured data present in an RDFa document. Processing need not follow the DOM traversal technique outlined here, although the effect of following some other manner of processing must be the same as if the processing outlined here were followed. The processing model is explained using the idea of DOM traversal which makes it easier to describe (particularly in relation to the evaluation context).

Note that in this section, explanations about the processing model or guidance to implementors are enclosed in sections like this.

7.1 Overview

Evaluating a document for RDFa triples is carried out by starting at the document object, and then visiting each of its child elements in turn, in document order, applying processing rules. Processing is recursive in that for each child element the processor also visits each of its child elements, and applies the same processing rules.

In some environments there will be little difference between starting at the root element of the document, and starting at the document object itself. It is defined this way because in some environments important information is present at the document object level which is not present on the root element.

As processing continues, rules are applied which may generate triples, and may also change theevaluation context information that will then be used when processing descendant elements.

This specification does not say anything about what should happen to the triples generated, or whether more triples might be generated during processing than are outlined here. However, to be conforming, an RDFa Processor must act as if at a minimum the rules in this section are applied, and a singleRDF graph produced. As described in the RDFa Processor Conformance section, any additional triples generated must not appear in the default graph.

7.2 Evaluation Context

During processing, each rule is applied using information provided by an evaluation context. An initial context is created when processing begins. That context has the following members:

• The base. This will usually be the URI of the document being processed, but it could be some other URI, set by some other mechanism, such as the (X)HTML base element. The important thing is that it establishes a URI against which relative paths can be resolved.
• The parent subject. The initial value will be the same as the initial value of base, but it will usually change during the course of processing.
• The parent object. In some situations the object of a statement becomes the subject of any nested statements, and this property is used to convey this value. Note that this value may be a bnode, since in some situations a number of nested statements are grouped together on one bnode. This means that the bnode must be set in the containing statement and passed down, and this property is used to convey this value.
• A list of current, in-scope URI mappings.
• A list of incomplete triples. A triple can be incomplete when no object resource is provided alongside a predicate that requires a resource (i.e., @rel or @rev). The triples can be completed when a resource becomes available, which will be when the next subject is specified (part of the process calledchaining).
• The language. Note that there is no default language.
• The term mappings, a list of terms and their associated URIs. This specification does not define an initial list. Host Languages may define an initial list. If a Host Language provides an initial list, it should do so via an RDFa Profile.
• The default vocabulary, a value to use as the prefix URI when a term is used. This specification does not define an initial setting for the default vocabulary. Host Languages may define an initial setting. If a Host Language defines an initial setting, it should do so via an RDFa Profile document.

During the course of processing, new evaluation contexts are created which are passed to each child element. The rules described below will determine the values of the items in the context. Additionally, some rules will cause new triples to be created by combining information provided by an element with information from the evaluation context.

During the course of processing a number of locally scoped values are needed, as follows:

• An initially empty list of URI mappings, called the local list of URI mappings.
• An initially empty list of incomplete triples, called the local list of incomplete triples.
• An initially empty language value.
• A skip element flag, which indicates whether the current element can safely be ignored since it has no relevant RDFa attributes. Note that descendant elements will still be processed.
• A new subject value, which once calculated will set the parent subject property in an evaluation context, as well as being used to complete any incomplete triples, as described in the next section.
• A value for the current object literal, the literal to use when creating triples that have a literal object.
• A value for the current object resource, the resource to use when creating triples that have a resource object.
• The local term mappings, a list of terms and their associated URIs.
• A local default vocabulary, a URI to use as a prefix mapping when a term is used.

7.3 Chaining

Statement chaining is an RDFa feature that allows the author to link RDF statements together while avoiding unnecessary repetitive markup. For example, if an author were to add statements as children of an object that was a resource, these statements should be interpreted as being about that resource:

In this example we can see that an object resource ('German_Empire'), has become the subject for nested statements. This markup also illustrates the basic chaining pattern of 'A has a B has a C' (i.e., Einstein has a birth place of the German Empire, which has a long name of "the German Empire").

It's also possible for the subject of nested statements to provide the object for containing statements — essentially the reverse of the example we have just seen. To illustrate, we'll take an example of the type of chaining just described, and show how it could be marked up more efficiently. To start, we mark up the fact that Albert Einstein had, at some point in his life, a residence both in the German Empire and in Switzerland:

Now, we show the same information, but this time we create an incomplete triple from the residence part, and then use any number of further subjects to 'complete' that triple, as follows:

In this example, the incomplete triple actually gets completed twice, once for the German Empire and once for Switzerland, giving exactly the same information as we had in the earlier example:

http://dbpedia.org/resource/Albert_Einstein dbp-owl:residence http://dbpedia.org/resource/German_Empire . http://dbpedia.org/resource/Albert_Einstein dbp-owl:residence http://dbpedia.org/resource/Switzerland .

Chaining can sometimes involve elements containing relatively minimal markup, for example showing only one resource, or only one predicate. Here the img element is used to carry a picture of Einstein:

When such minimal markup is used, any of the resource-related attributes could act as a subject or an object in the chaining:

7.4 CURIE and URI Processing

Since RDFa is ultimately a means for transporting RDF, a key concept is the resource and its manifestation as a URI. RDF deals with complete URIs (not relative paths); when converting RDFa to triples, any relative URIs must be resolved relative to the base URI, using the algorithm defined in section 5 of RFC 3986 [URI],Reference Resolution. The values of RDFa attributes that refer to URIs use three different datatypes: URI, SafeCURIEorCURIEorURI, or TERMorCURIEorAbsURI. All these attributes are mapped, after processing, to URIs. The handling of these attributes is as follows:

URI

The content is a URI, and is used as such.

SafeCURIEorCURIEorURI

• When the value is surrounded by square brackets, then the content within the brackets is evaluated as a CURIE according to the CURIE Syntax definition. If it is not a valid CURIE, the value must be ignored.
• Otherwise, the value is evaluated as a CURIE. If it is a valid CURIE, the resulting URI is used; otherwise, the value is processed as a URI.

TERMorCURIEorAbsURI

• If the value is an NCName, then it is evaluated as a term according to General Use of Terms in Attributes. Note that this step may mean that the value is to be ignored.
• If the value is a valid CURIE, then the resulting URI is used.
• If the value is an absolute URI, that value is used.
• Otherwise, the value is ignored.

Note that it is possible for all values in an attribute to be ignored. When that happens, the attribute must be treated as if it were empty.

For example, the full URI for Albert Einstein on DBPedia is:

http://dbpedia.org/resource/Albert_Einstein

This can be shortened by authors to make the information easier to manage, using a CURIE. The first step is for the author to create a prefix mapping that links a prefix to some leading segment of the URI. In RDFa these mappings are expressed using the XML namespace syntax:

Once the prefix has been established, an author can then use it to shorten a URI as follows:

The author is free to split the URI at any point, as long as it begins at the left end. However, since a common use of CURIEs is to make available libraries of terms and values, the prefix will usually be mapped to some common segment that provides the most re-use, often provided by those who manage the library of terms. For example, since DBPedia contains an enormous list of resources, it is more efficient to create a prefix mapping that uses the base location of the resources:

Note that it is generally considered a bad idea to use relative paths in prefix declarations. Since it is possible that an author may ignore this guidance, it is further possible that the URI obtained from a CURIE is relative. However, since all URIs must be resolved relative to base before being used to create triples, the use of relative paths should not have any effect on processing.

7.4.1 Scoping of Prefix Mappings

CURIE prefix mappings are defined on the current element and its descendants. The inner-most mapping for a given prefix takes precedence. For example, the URIs expressed by the following two CURIEs are different, despite the common prefix, because the prefix mappings are locally scoped:

7.4.2 General Use of CURIEs in Attributes

There are a number of ways that attributes make use of CURIEs, and they need to be dealt with differently. These are:

1. An attribute may allow one or more values that are a mixture of TERMs, CURIEs, and absolute URIs.
2. An attribute may allow one or more values that are a mixture of CURIEs and URIs. In this case any value that is not a CURIE, as outlined in section CURIE Syntax Definition, will be processed as a URI.
3. If the value is surrounded by square brackets, then the content within the brackets is always evaluated according to the rules in CURIE Syntax Definition - and if that content is not a CURIE, then the content must be ignored.

An empty attribute value (e.g., typeof='') is still a CURIE, and is processed as such. The rules for this processing are defined in Sequence. Specifically, however, an empty attribute value is never treated as a relative URI by this specification.

An example of an attribute that can contain a CURIEorURI is@about. To express a URI directly, an author might do this:

whilst to express the URI above as a CURIE they would do this:

The author could also use a safe CURIE, as follows:

Since non-CURIE values must be ignored, the following value in @about would not set a new subject, since @about does not permit the use of TERMs, and the CURIE has no prefix separator.

However, this markup would set a subject, since it is not a CURIE, but a valid relative URI:

Note that several RDFa attributes are able to also take TERMS as their value. This is discussed in the next section.

7.4.3 General Use of Terms in Attributes

Some RDFa attributes have a datatype that permits a term to be referenced. RDFa defines the syntax of a term as:

term ::= NCName

When an RDFa attribute permits the use of a term, and the value being evaluated matches the production for term above, it is transformed to a URI using the following logic:

• Check if the term matches an item in the list of local term mappings. First compare against the list_case-sensitively_, and if there is no match then compare case-insensitively. If there is a match, use the associated URI.
• Otherwise, if there is a local default vocabulary the URI is obtained by concatenating that value and the term.
• Finally, if there is no local default vocabulary, the term has no associated URI and must be ignored.

In the event that multiple terms are defined that differ only in case (e.g., 'Agent', 'agent', and 'AGENT'), if a reference is made which DOES NOT match case-sensitively (e.g., typeof='AGENt'), the results are UNSPECIFIED.

7.4.4 Use of CURIEs in Specific Attributes

The general rules discussed in the previous sections apply to the RDFa attributes in the following ways:

• @about and @resource support the datatypeSafeCURIEorCURIEorURI - allowing a SafeCURIE, a CURIE, or a URI.
• @href and @src are as defined in the Host Language (e.g., XHTML), and support only a URI.
• @profile also only supports a URI.
• @vocab supports a white space separated list of URIs.
• @datatype supports the datatype TERMorCURIEorAbsURI - allowing a single Term, CURIE, or Absolute URI.
• @property, @typeof, @rel, and @rev support the datatype TERMorCURIEorAbsURIs - allowing one or more Terms, CURIEs, or Absolute URIs.

Any value that matches a defined term must be expanded into a reference to the corresponding URI. For example in [XHTML-RDFA] the following examples:

would each generate the following triple:

<> http://www.w3.org/1999/xhtml/vocab#next http://example.org/page2.html .

7.4.5 Referencing Blank Nodes

In RDFa, it is possible to establish relationships using various types of resource references, including bnodes. If a subject or object is defined using a CURIE, and that CURIE explicitly names a bnode, then a Conforming Processor must create the bnode when it is encountered during parsing. The RDFa Processor must also ensure that no bnode created automatically (as a result of chaining) has a name that collides with a bnode that is defined by explicit reference in a CURIE.

Consider the following example:

In the above fragment, two bnodes are explicitly created as the subject of triples. Those bnodes are then referenced to demonstrate the relationship between the parties. After processing, the following triples will be generated:

_:john foaf:mbox mailto:john@example.org . _:sue foaf:mbox mailto:sue@example.org . _:john foaf:knows _:sue .

RDFa Processors use, internally, implementation-dependent identifiers for bnodes. When triples are retrieved, new bnode indentifiers are used, which usually bear no relation to the original identifiers. However, implementations do ensure that these generated bnode identifiers are consistent: each bnode will have its own identifier, all references to a particular bnode will use the same identifier, and different bnodes will have different identifiers.

As a special case, _: is also a valid reference for one specific bnode.

7.5 Sequence

Processing would normally begin after the document to be parsed has been completely loaded. However, there is no requirement for this to be the case, and it is certainly possible to use a stream-based approach, such as SAX [SAX] to extract the RDFa information. However, if some approach other than the DOM traversal technique defined here is used, it is important to ensure that Host Language-specific processing rules are applied (e.g., XHTML+RDFa [XHTML-RDFA] indicates the base element can be used, andbase will affect the interpretation of URIs in meta orlink elements even if those elements are before thebase element in the stream).

At the beginning of processing, an initial evaluation context is created, as follows:

• the base is set to the URI of the document (or another value specified in a language specific manner such as the HTMLbase element);
• the parent subject is set to the base value;
• the parent object is set to null;
• the list of incomplete triples is empty;
• the language is set to null.
• the list of URI mappings is empty (or a list defined in the Host Language-defined default RDFa Profile).
• the term mappings is set to null (or a list defined in the Host Language-defined default RDFa Profile).
• the default vocabulary is set to null (or a URI defined in the Host Language-defined default RDFa Profile).

Processing begins by applying the processing rules below to the document object, in the context of this initialevaluation context. All elements in the tree are also processed according to the rules described below, depth-first, although the evaluation context used for each set of rules will be based on previous rules that may have been applied.

This specification defines processing rules for optional attributes that may not be present in all Host Languages (e.g., @xmlns:). If these attributes are not supported in the Host Language, then the corresponding processing rules are not relevant for that language.

The processing rules are:

1. First, the local values are initialized, as follows:
   • the skip element flag is set to 'false';
   • new subject is set to null;
   • current object resource is set to null;
   • the local list of URI mappings is set to the list of URI mappings from the evaluation context;
   • the local list of incomplete triples is set to null;
   • the current language value is set to the language value from the evaluation context.
   • the local term mappings is set to the term mappings from the evaluation context.
   • the local default vocabulary is set to the default vocabulary from the evaluation context.
     Note that some of the local variables are temporary containers for values that will be passed to descendant elements via anevaluation context. In some cases the containers will have the same name, so to make it clear which is being acted upon in the following steps, the local version of an item will generally be referred to as such.
2. Next the current element is parsed for any updates to thelocal term mappings, default vocabulary, and local list of URI mappings via @profile. If@profile is present, its value is processed as defined in RDFa Profiles. If any referenced RDFa Profile is not recognized, then the current element and its children must not place any triples in the default graph.
   Any new terms or URI mappings are merged into respective local lists. They are in effect for this element and for its children.
3. Next the current element is examined for any change to thedefault vocabulary via @vocab. If @vocab is present and contains a value, its value updates the local default vocabulary. If the value is empty, then the local default vocabulary must be reset to the Host Language defined default.
   A Host Language is not required to define a default vocabulary. In such a case, setting @vocab to the empty value has the effect of clearing the local default vocabulary.
4. Next, the current element is examined for URI mappings and these are added to the local list of URI mappings. Note that a URI mapping will simply overwrite any current mapping in the list that has the same name;
   Mappings are defined via @prefix. Values in this attribute are evaluated from beginning to end (e.g., left to right in typical documents). For backward compatibility, RDFa Processors should also permit the definition of mappings via @xmlns. In this case, the value to be mapped is set by the XML namespace prefix, and the value to map is the value of the attribute — a URI. (Note that prefix mapping via @xmlns is deprecated, and may be removed in a future version of this specification.) When xmlns is supported, such mappings must be processed before processing an mappings from @prefix on the same element. Regardless of how the mapping is declared, the value to be mapped must be converted to lower case, and the URI is not processed in any way; in particular if it is a relative path it must not be resolved against the current base. Authors should not use relative paths as the URI.
5. The current element is also parsed for any language information, and if present, current language is set accordingly;
   Host Languages that incorporate RDFa may provide a mechanism for specifying the natural language of an element and its contents (e.g., XML provides the general-purpose XML attribute @xml:lang).
6. If the current element contains no @rel or @rev attribute, then the next step is to establish a value for new subject. Any of the attributes that can carry a resource can set new subject;
   new subject is set to the URI obtained from the first match from the following rules:
   • by using the URI from @about, if present, obtained according to the section onCURIE and URI Processing;
   • otherwise, by using the URI from @src, if present, obtained according to the section onCURIE and URI Processing.
   • otherwise, by using the URI from @resource, if present, obtained according to the section onCURIE and URI Processing;
   • otherwise, by using the URI from @href, if present, obtained according to the section onCURIE and URI Processing.
     If no URI is provided by a resource attribute, then the first match from the following rules will apply:
   • if the element is the root element of the document, then act as if there is an empty @about present, and process it according to the rule for @about, above;
   • if @typeof is present, then new subject is set to be a newly created bnode.
   • otherwise, if parent object is present, new subject is set to the value ofparent object. Additionally, if @property is not present then the skip element flag is set to 'true';
7. If the current element does contain a @rel or @rev attribute, then the next step is to establish both a value for new subject and a value for current object resource:
   new subject is set to the URI obtained from the first match from the following rules:
   • by using the URI from @about, if present, obtained according to the section onCURIE and URI Processing;
   • otherwise, by using the URI from @src, if present, obtained according to the section onCURIE and URI Processing.
     If no URI is provided then the first match from the following rules will apply:
   • if the element is the root element of the document then act as if there is an empty @about present, and process it according to the rule for @about, above;
   • if @typeof is present, then new subject is set to be a newly created bnode;
   • otherwise, if parent object is present, new subject is set to that.
     Then the current object resource is set to the URI obtained from the first match from the following rules:
   • by using the URI from @resource, if present, obtained according to the section onCURIE and URI Processing;
   • otherwise, by using the URI from @href, if present, obtained according to the section onCURIE and URI Processing.
     Note that final value of the current object resource will either be null (from initialization) or a full URI.
8. If in any of the previous steps a new subject was set to a non-null value, it is now used to provide a subject for type values;
   One or more 'types' for the new subject can be set by using @typeof. If present, the attribute may contain one or more URIs, obtained according to the section onURI and CURIE Processing, each of which is used to generate a triple as follows:
   subject
   new subject
   predicate
   http://www.w3.org/1999/02/22-rdf-syntax-ns#type
   object
   full URI of 'type'
   Note that none of this block is executed if there is no new subject value, i.e.,new subject remains null.
9. If in any of the previous steps a current object resource was set to a non-null value, it is now used to generate triples:
   Predicates for the current object resource can be set by using one or both of the @rel and@rev attributes:
   • If present, @rel may contain one or more URIs, obtained according to the section on CURIE and URI Processing each of which is used to generate a triple as follows:
     subject
     new subject
     predicate
     full URI
     object
     current object resource
   • If present, @rev may contain one or more URIs, obtained according to the section onCURIE and URI Processing each of which is used to generate a triple as follows:
     subject
     current object resource
     predicate
     full URI
     object
     new subject
10. If however current object resource was set to null, but there are predicates present, then they must be stored as incomplete triples, pending the discovery of a subject that can be used as the object. Also, current object resource should be set to a newly createdbnode;
    Predicates for incomplete triples can be set by using one or both of the @rel and@rev attributes:

• If present, @rel must contain one or more URIs, obtained according to the section on CURIE and URI Processing each of which is added to the local list of incomplete triples as follows:
  predicate
  full URI
  direction
  forward
• If present, @rev must contain one or more URIs, obtained according to the section onCURIE and URI Processing, each of which is added to thelocal list of incomplete triples as follows:
  predicate
  full URI
  direction
  reverse

1. The next step of the iteration is to establish any current object literal;
   Predicates for the current object literal can be set by using @property. If present, one or more URIs are obtained according to the section on CURIE and URI Processing, and then the actual literal value is obtained as follows:

• as a typed literal if @datatype is present, does not have an empty value according to the section on CURIE and URI Processing, and is not set to XMLLiteral in the vocabulary http://www.w3.org/1999/02/22-rdf-syntax-ns#.
  The actual literal is either the value of @content (if present) or a string created by concatenating the value of all descendant text nodes, of the current element in turn. The final string includes the datatype URI, as described in [RDF-CONCEPTS], which will have been obtained according to the section onCURIE and URI Processing.
• as an XML literal if @datatype is present and is set to XMLLiteral in the vocabulary http://www.w3.org/1999/02/22-rdf-syntax-ns#.
  The value of the XML literal is a string created by serializing to text, all nodes that are descendants of the current element, i.e., not including the element itself, and giving it a datatype ofXMLLiteral in the vocabulary http://www.w3.org/1999/02/22-rdf-syntax-ns#. The format of the resulting serialized content is as defined in Exclusive XML Canonicalization Version [XML-EXC-C14N].
  In order to maintain maximum portability of this literal, any children of the current node that are elements must have the current XML namespace declarations (if any) declared on the serialized element. Since the child element node could also declare new XML namespaces, the RDFa Processor must be careful to merge these together when generating the serialized element definition. For avoidance of doubt, any re-declarations on the child node must take precedence over declarations that were active on the current node.
• otherwise as a plain literal.
  Additionally, if there is a value for current language then the value of the plain literal should include this language information, as described in [RDF-CONCEPTS]. The actual literal is either the value of@content (if present)or a string created by concatenating the text content of each of the descendant elements of the current element in document order.
  The current object literal is then used with each predicate to generate a triple as follows:
  subject
  new subject
  predicate
  full URI
  object
  current object literal

1. If the skip element flag is 'false', and new subject was set to a non-null value, then anyincomplete triples within the current context should be completed:
   The list of incomplete triples from the current evaluation context (not thelocal list of incomplete triples) will contain zero or more predicate URIs. This list is iterated, and each of the predicates is used with parent subject and new subject to generate a triple. Note that at each level there are two lists ofincomplete triples; one for the current processing level (which is passed to each child element in the previous step), and one that was received as part of the evaluation context. It is the latter that is used in processing during this step.
   Note that each incomplete triple has a direction value that it used to determine what will become the subject, and what will become the object, of each generated triple:

• If direction is 'forward' then the following triple is generated:
  subject
  parent subject
  predicate
  the predicate from the iterated incomplete triple
  object
  new subject
• If direction is not 'forward' then this is the triple generated:
  subject
  new subject
  predicate
  the predicate from the iterated incomplete triple
  object
  parent subject

1. Next, all elements that are children of the current element are processed using the rules described here, using a new evaluation context, initialized as follows:

• If the skip element flag is 'true' then the new evaluation context is a copy of the current context that was passed in to this level of processing, with the language and list of URI mappings values replaced with the local values;
• Otherwise, the values are:
  * the base is set to the base value of the current evaluation context;
  * the parent subject is set to the value of new subject, if non-null, or the value of the parent subject of the current evaluation context;
  * the parent object is set to value of current object resource, if non-null,or the value of new subject, if non-null, or the value of theparent subject of the current evaluation context;
  * the list of URI mappings is set to the local list of URI mappings;
  * the list of incomplete triples is set to the local list of incomplete triples;
  * language is set to the value of current language.
  * the term mappings is set to the value of thelocal term mappings.
  * the default vocabulary is set to the value of thelocal default vocabulary.

7.6 Processor Status

The processing rules covered in the previous section are designed to extract as many triples as possible from a document. The RDFa Processor is designed to continue processing, even in the event of errors. For example, failing to resolve a prefix mapping or term would result in the RDFa Processor skipping the generation of a triple and continuing with document processing. There are cases where knowing each RDFa Processor warning or error would be beneficial to authors. Theprocessor graph is designed as a possible mechanism to capture all informational, warning, and error messages as triples from the RDFa Processor. These status triples may be retrieved and used to aid RDFa authoring or automated error detection.

If an RDFa Processor supports the generation of a processor graph, then it must generate a set of triples when the following processing issues occur:

• An rdfa:Error must be generated when the document fails to be fully processed as a result of non-conformant host language markup.
• An rdfa:Error must be generated when a referenced RDFa Profile is notrecognized, as described in RDFa Profiles.
• A rdfa:Warning must be generated when a CURIE prefix fails to be resolved.
• A rdfa:Warning must be generated when a Term fails to be resolved.

Other implementation-specific rdfa:Info, rdfa:Warning, or rdfa:Error triples may be generated by the RDFa Processor.

7.6.1 Accessing the Processor Graph

Accessing the processor graph may be accomplished in a variety of ways and is dependent on the type of RDFa Processor and access method that the developer is utilizing.

SAX-based processors or processors that utilize function or method callbacks to report the generation of triples are classified asevent-based RDFa Processors. For Event-based RDFa Processors, the software must allow the developer to register a function or callback that is called when a triple is generated for theprocessor graph. The callback may be the same as the one that is used for the default graph as long as it can be determined if a generated triple belongs in the processor graph or the default graph.

A whole-graph RDFa Processor is defined as any RDFa Processor that processes the entire document and only allows developer access to the triples after processing has completed. RDFa Processors that typically fall into this category express their output via a single call using RDF/XML, N3, TURTLE, or N-Triples notation. For whole-graph RDFa Processors, the software must allow the developer to specify if they would like to retrieve the default graph, the processor graph, or both graphs as a single, combined graph from the RDFa Processor. If the graph preference is not specified, the default graph must be returned.

An web service RDFa Processor is defined as any RDFa Processor that is capable of processing a document by performing an HTTP GET, POST or similar action on an RDFa Processor URI. For this class of RDFa Processor, the software must allow the caller to specify if they would like to retrieve the default graph, the processor graph, or both graphs as a single, combined graph from the web service. The rdfagraph query parameter must be used to specify the value. The allowable values aredefault, processor or both values, in any order, separated by a comma character. If the graph preference is not specified, the default graph must be returned.

7.6.2 Processor Graph Terms

To ensure interoperability, a core hierarchy of classes is defined for the content of the processor graph. Separate errors or warnings are resources (typically blank nodes) of a specific type, with additional properties giving more details on the error condition or the warning. This specification defines only the top level classes and the ones referring to the error and warning conditions defined explicitly by this document. Other, implementation-specific subclasses may be defined by the RDFa Processor.

The top level classes are rdfa:Error, rdfa:Warning, and rdfa:Info, defined as part of the RDFa Vocabulary. Furthermore, a single property is defined on those classes, namely rdfa:context, that provides an extra context for the error, e.g., http response, an XPath information, or simply the URI to the RDFa resource. Usage of this property is optional, and more than one triple can be used with this predicate on the same subject. Finally, error and warning instances should use the dc:description and dc:date properties. dc:description should provide a short, human readable but implementation dependent description of the error. dc:date should give the time when the error was found and it is advised to be as precise as possible to allow the detection of, for example, possible network errors.

The example below shows the triples that should be minimally present in the processor graph as a result of an error (the content of the literal for the dc:description predicate is implementation dependent):

@prefix rdfa: http://www.w3.org/ns/rdfa# . @prefix xsd: http://www.w3.org/2001/XMLSchema# . @prefix dcterms: http://purl.org/dc/terms/ . [] a rdfa:ProfileReferenceError ; dcterms:description "The @profile value could not be deferenced" ; dcterms:date "2010-06-30T13:40:23"^^xsd:dateTime .

A slightly more elaborate example makes use of the rdfa:context property to provide further information, using external vocabularies to represent HTTP headers or XPointer information:

@prefix rdfa: http://www.w3.org/ns/rdfa# . @prefix xsd: http://www.w3.org/2001/XMLSchema# . @prefix dcterms: http://purl.org/dc/terms/ . @prefix ptr: http://www.w3.org/2009/pointers# . @prefix http: http://www.w3.org/2006/http# .

[] a rdfa:ProfileReferenceError ; dcterms:description "The @profile value could not be deferenced" ; dcterms:date "2010-06-30T13:40:23"^^xsd:dateTime ; rdfa:context http://www.example.org/profile ; rdfa:context [ a ptr:Pointer ; # detailed xpointer/xpath information provided here to locate the # DOM node with the @profile attribute ] ; rdfa:context [ a http:Response ; http:responseCode http://www.w3.org/2006/http#404 # Get the HTTP response headers on the request for the profile file. ].

8. RDFa Processing in detail

This section is non-normative.

This section provides an in-depth examination of the processing steps described in the previous section. It also includes examples which may help clarify some of the steps involved.

The key to processing is that a triple is generated whenever a predicate/object combination is detected. The actual triple generated will include a subject that may have been set previously, so this is tracked in the current evaluation context and is called the parent subject. Since the subject will default to the current document if it hasn't been set explicitly, then a predicate/object combination is always enough to generate one or more triples.

The attributes for setting a predicate are @rel, @rev and @property, whilst the attributes for setting an object are @resource, @href, @content, and @src.@typeof is unique in that it sets both a predicate and an object at the same time (and also a subject when it appears in the absence of other attributes that would set a subject). Inline content might also set an object, if @content is not present, but @property is present.

There are many examples in this section. The examples are all written using XHTML+RDFa. However, the explanations are relevant regardless of the Host Language.

8.1 Changing the evaluation context

8.1.1 Setting the current subject

When triples are created they will always be in relation to a subject resource which is provided either by new subject (if there are rules on the current element that have set a subject) or parent subject, as passed in via theevaluation context. This section looks at the specific ways in which these values are set. Note that it doesn't matter how the subject is set, so in this section we use the idea of the current subject which may be either new subject or parent subject.

8.1.1.1 The current document

When parsing begins, the current subject will be the URI of the document being parsed, or a value as set by a Host Language-provided mechanism (e.g., the base element in (X)HTML). This means that by default any metadata found in the document will concern the document itself:

This would generate the following triples:

<> foaf:primaryTopic <#bbq> . <> dcterms:creator "Jo" .

It is possible for the data to appear elsewhere in the document:

Jo's blog

Welcome to my blog.

which would still generate the triple:

<> dcterms:creator "Jo" .

In (X)HTML the value of base may change the initial value of current subject:

An RDFa Processor should now generate the following triples, regardless of the URI from which the document is served:

http://www.example.org/jo/blog foaf:primaryTopic <#bbq> . http://www.example.org/jo/blog dcterms:creator "Jo" .

8.1.1.2 Using @about

As processing progresses, any@about attributes will change the current subject. The value of@about is a URI or a CURIE. If it is a relative URI then it needs to be resolved against the current base value. To illustrate how this affects the statements, note in this markup how the properties inside the (X)HTML body element become part of a new calendar event object, rather than referring to the document as they do in the head of the document:

I'm holding one last summer barbecue , on September 16th at 4pm .

With this markup an RDFa Processor will generate the following triples:

<> foaf:primaryTopic <#bbq> . <> dcterms:creator "Jo" . <#bbq> rdf:type cal:Vevent . <#bbq> cal:summary "one last summer barbecue" . <#bbq> cal:dtstart "2015-09-16T16:00:00-05:00"^^xsd:dateTime .

Other kinds of resources can be used to set the current subject, not just references to web-pages. Although not advised, email addresses might be used to represent a person:

John knows Sue.

Sue knows Jim.

This should generate the following triples:

mailto:john@example.org foaf:knows mailto:sue@example.org . mailto:sue@example.org foaf:knows mailto:jim@example.org .

Similarly, authors may make statements about images:

which should generate the following triples:

<photo1.jpg> dcterms:creator "Mark Birbeck" .

8.1.1.3 Using @src

If @about is not present, then @src is next in priority order, for setting the subject of a statement. A typical use would be to indicate the licensing type of an image:

Since there is no difference between @src and @about, then the information expressed in the last example in the section on @about (the creator of an image), could be expressed as follows:

Since normal chaining rules will apply, the image URI can also be used to complete hanging triples:

The complete markup yields three triples:

http://www.blogger.com/profile/1109404 foaf:img <photo1.jpg> . <photo1.jpg> xhv:license http://creativecommons.org/licenses/by/2.0/ . <photo1.jpg> dcterms:creator "Mark Birbeck" .

8.1.1.4 Creating a new item with @typeof

Whilst @about explicitly creates a new context for statements, @typeof does so implicitly.@typeof works differently to other ways of setting a predicate since the predicate is alwaysrdf:type, which means that the processor only requires one attribute, the value of the type.

Since @typeof is setting the type of an item, this means that if no item exists one should automatically be created. This involves generating a new bnode, and is examined in more detail below; it is mentioned here because the bnode used by the new item will become the subject for further statements.

For example, an author may wish to create markup for a person using the FOAF vocabulary, but without having a clear identifier for the item:

This markup would cause a bnode to be created which has a 'type' of foaf:Person, as well as name and given name properties:

_:a rdf:type foaf:Person . _:a foaf:name "Albert Einstein" . _:a foaf:givenName "Albert" .

A bnode is simply a unique identifier that is only available to the processor, not to any external software. By generating values internally, the processor is able to keep track of properties for _:a as being distinct from_:b. But by not exposing these values to any external software, it is possible to have complete control over the identifier, as well as preventing further statements being made about the item.

8.1.1.5 Determining the subject with neither @about nor @typeof

As described in the previous two sections, @about will always take precedence and mark a new subject, but if no@about value is available then @typeof will do the same job, although using an implied identifier, i.e., a bnode.

But if neither @about or @typeof are present, there are a number of ways that the subject could be arrived at. One of these is to 'inherit' the subject from the containing statement, with the value to be inherited set either explicitly, or implicitly.

8.1.1.5.1 Inheriting subject from @resource

The most usual way that an inherited subject might get set would be when the parent statement has an object that is a resource. Returning to the earlier example, in which the long name for the German_Empire was added, the following markup was used:

<xs:simpleType name="CURIE">
    <xs:restriction base="xs:string">
        <xs:pattern value="(([\i-[:]][\c-[:]]*)?:)?.+" />
        <xs:minLength value="1"/>
    </xs:restriction>
</xs:simpleType> 

<xs:simpleType name="CURIEs">
    <xs:list itemType="xh11d:CURIE"/>
</xs:simpleType>

<xs:simpleType name="SafeCURIE">
    <xs:restriction base="xs:string">
        <xs:pattern value="\[(([\i-[:]][\c-[:]]*)?:)?.+\]" />
        <xs:minLength value="3"/>
    </xs:restriction>
</xs:simpleType>

<xs:simpleType name="SafeCURIEs">
    <xs:list itemType="xh11d:SafeCURIE"/>
</xs:simpleType>

<xs:simpleType name="TERM">
    <xs:list itemType="xs:NCName"/>
</xs:simpleType>

<xs:simpleType name="CURIEorURI">
    <xs:union memberTypes="xh11d:CURIE xs:anyURI" />
</xs:simpleType>

<xs:simpleType name="CURIEorURIs">
    <xs:list itemType="xh11d:CURIEorURI"/>
</xs:simpleType>

<xs:simpleType name="SafeCURIEorCURIEorURI">
    <xs:union memberTypes="xh11d:SafeCURIE xh11d:CURIE xs:anyURI" />
</xs:simpleType>

<xs:simpleType name="SafeCURIEorCURIEorURIs">
    <xs:list itemType="xh11d:SafeCURIEorCURIEorURI"/>
</xs:simpleType>

<xs:simpleType name='AbsURI'>
    <xs:restriction base='xs:string'>
        <xs:pattern value="[\i-[:]][\c-[:]]+:.+" />
    </xs:restriction>
</xs:simpleType>

<xs:simpleType name="TERMorCURIEorAbsURI">
    <xs:union memberTypes="xh11d:TERM xh11d:CURIE xh11d:AbsURI" />
</xs:simpleType>

<xs:simpleType name="TERMorCURIEorAbsURIs">
    <xs:list itemType="xh11d:SafeCURIEorCURIEorAbsURI"/>
</xs:simpleType>