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Copyright © 2007 W3C® (MIT, ERCIM, Keio), All Rights Reserved. W3C liability, trademark and document use rules apply.
The modern Web is 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 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 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 intere with each other.
This document is a detailed syntax specification for RDFa, aimed at:
If you are already familiar with RDFa, and you want to examine the processing rules--perhaps to create a parser--then you'll find section x of most interest. Each of the processing steps in this section is outlined in more detail in the sections after it.
If you are not familiar with RDFa, but you are familiar with RDF, then you might find reading the overview useful, since it gives a range of examples of XHTML mark-up 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 section y before 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 on RDF on the web, and a growing range of tools that will support RDFa, so all we try to do in this document is provide enough background on RDF to make the goals of RDFa clearer.
And finally, 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 to be a better introduction.
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 jointly by the Semantic Web Deployment Working Group [SWD-WG] and the XHTML 2 Working Group [XHTML2-WG]. Initial work on RDFa began in the XHTML 2 Working Group [XHTML2-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.
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 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, with minimal repetition of data.
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." The user experience could then be enhanced, for example by providing contextual information about specific rendered data, perhaps when the user "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 [HTML4] there is a cite
attribute; 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 a parser 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 generic rather than hard-wired manner. This has been achieved by creating a fixed set of attributes and parsing rules, but allowing those attibutes to contain properties from any of a number of the growing range of available taxonomies. The values of those properties are in most cases the information that is already in an author's document.
RDFa takes the pressure off language authors to anticipate all the structural requirements users of their language might have, by outlining a new syntax for RDF that relies only on attributes. RDFa can be easily imported into other XML-based markup languages, as well as HTML, allowing any mark-up language to carry arbitrary RDF.
This specification deals specifically with the use of RDFa in HTML-based languages, such as HTML and XHTML, and defines an RDF mapping for a number of HTML attributes.
NOTE: The idea of this section is to give the reader a quick intro to the features RDFa supports. It's not meant to be exhaustive, but if I have missed anything obvious then do let me know. The idea is to use a simple example, and to make them 'familiar', which is why I chose to use the mark-up from the Primer.
The following examples are intended to help readers who are not familiar with RDFa to quickly get a sense of how it works.
As an HTML author you will already be familiar with using meta
and link
to add additional information to your documents:
<html xmlns="http://www.w3.org/1999/xhtml"> <head> <title>Page 7</title> <meta name="author" content="Mark Birbeck" /> <link rel="prev" href="page6.html" /> <link rel="next" href="page8.html" /> </head> <body>...</body> </html>
RDFa makes uses of this concept, enhancing it with the ability to make use of other vocabularies by using namespaces:
<html xmlns="http://www.w3.org/1999/xhtml" xmlns:foaf="http://xmlns.com/foaf/0.1/" xmlns:dc="http://purl.org/dc/elements/1.1/" > <head> <title>My home-page</title> <meta property="dc:creator" content="Mark Birbeck" /> <link rel="foaf:workplaceHomepage" href="http://www.formsPlayer.com/" /> </head> <body>...</body> </html>
Although not widely used, HTML already supports the use of @rel
and @rev
on the a
element. This becomes more useful in RDFa with the addition of namespace
support:
This document is licensed under a <a xmlns:cclicenses="http://creativecommons.org/licenses/" rel="cc:license" href="http://creativecommons.org/licenses/by/nc-nd/3.0/"> Creative Commons License </a>.
Not only can URLs in the document be re-used to provide metadata, but so can inline text:
<html xmlns="http://www.w3.org/1999/xhtml" xmlns:cal="http://www.w3.org/2002/12/cal/ical#" xmlns:xs="http://www.w3.org/2001/XMLSchema" > <head><title>Jo's Friends and Family Blog</title></head> <body> <p> I'm holding <span property="cal:summary"> one last summer Barbecue </span>, on September 16th at 4pm. </p> </body> </html>
If some displayed text is different to the actual 'value' it represents, more precise values can be added, which can optionally include datatypes:
<html xmlns="http://www.w3.org/1999/xhtml" xmlns:cal="http://www.w3.org/2002/12/cal/ical#" xmlns:xs="http://www.w3.org/2001/XMLSchema" > <head><title>Jo's Friends and Family Blog</title></head> <body> <p> I'm holding <span property="cal:summary"> one last summer Barbecue </span>, on <span property="cal:dtstart" content="20070916T1600-0500" datatype="xs:datetime"> September 16th at 4pm </span>. </p> </body> </html>
In many cases a block of mark-up will contain a number of properties that relate to the same item; it's possible with RDFa to indicate the type of that item:
<html xmlns="http://www.w3.org/1999/xhtml" xmlns:cal="http://www.w3.org/2002/12/cal/ical#" xmlns:xs="http://www.w3.org/2001/XMLSchema" > <head><title>Jo's Friends and Family Blog</title></head> <body> <p instanceof="cal:Vevent"> I'm holding <span property="cal:summary"> one last summer Barbecue </span>, on <span property="cal:dtstart" content="20070916T1600-0500" datatype="xs:datetime"> September 16th at 4pm </span>. </p> </body> </html>
The metadata features available in HTML only allow information to be expressed about the document itself. RDFa provides a means of referring to other documents and resources:
<html xmlns="http://www.w3.org/1999/xhtml" xmlns:bib="http://somebig.org/" > <head> <title>Books by Marco Pierre White</title> </head> <body> I think <span about="urn:ISBN:0091808189" instanceof="bib:book">White's book 'Canteen Cuisine'</span> is well worth getting since although it's quite advanced stuff, he makes it pretty easy to follow. You might also like his <span about="urn:ISBN:1596913614" instanceof="bib:book">autobiography</span>. </body> </html>
The previous section gave examples of typical mark-up in order to illustrate what RDFa in XHTML looks like. But what RDFa in XHTML represents is RDF. In order to author RDFa in XHTML you do not need to understood RDF at all, although it would certainly help. However, if you are building a system that consumes the RDF output of an RDFa in XHTML document you will obviously need to understand RDF--but you can also skip this section, since here we provide a very basic introduction to RDF terminology so that we can make use of it in the following sections.
The metadata information that RDFa provides access to is generally understood to be a collection of statements. A statement is a basic unit of information that has been constructed in a very specific way 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 made available for processing.
To illustrate, suppose we have the following set of facts:
Albert was born on March 14, 1879, in Germany. There is a picture of him at http://en.wikipedia.org/wiki/Image:Albert_Einstein_Head.jpg.This would be quite difficult for a machine to process, and it is certainly not in a format that could be passed from one system 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 as follows:
Albert was born on March 14, 1879. Albert was born in Germany. Albert has a picture at http://en.wikipedia.org/wiki/Image:Albert_Einstein_Head.jpg.
To make this information machine-processable RDF defines the structure of these statements very tightly. A statement is actually a triple, meaning that it is made up three components. The first is the subject of the statement, and is what we are making our statements about. In these examples the subject is always '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 the value of the property, or the object. In the examples here the object values are 'March 14, 1879', 'Germany', and 'http://en.wikipedia.org/wiki/Image:Albert_Einstein_Head.jpg'.
Breaking complex information into manageable units obviously helps, but there is still ambiguity here. For example, which 'Albert' are we talking about? If some system has further facts--triples--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? Also, if we wanted to find people born in Germany, how could we know that the predicate 'was born in' has the same purpose as the predicate 'birthplace' that exists 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 by using the DBPedia URI for Albert Einstein:
<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 Germany. <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 (note that the picture of Einstein is already a URI):
<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/Germany>. <http://dbpedia.org/resource/Albert_Einstein> has a picture at <http://en.wikipedia.org/wiki/Image:Albert_Einstein_Head.jpg>.
And of course URI references are also used to ensure that predicates are unambiguous:
<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/Germany>. <http://dbpedia.org/resource/Albert_Einstein> <http://xmlns.com/foaf/0.1/depiction> <http://en.wikipedia.org/wiki/Image:Albert_Einstein_Head.jpg>.
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".
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 which indicates the literal's datatype. This URI is usually based on datatypes defined in the XML Schema Datatypes specification [XML SCHEMA DATATYPES REFERENCE /
http://www.w3.org/TR/xmlschema-2/]. The following syntax would be used to unambiguously express Einstein's date of birth as a literal of type xsd:date
:
<http://dbpedia.org/resource/Albert_Einstein> <http://dbpedia.org/property/dateOfBirth> "1879-03-14"^^<http://www.w3.org/2001/XMLSchema#date>.
RDF does not have one set way to express triples, since the key ideas of RDF are the triple and the use of URIs. However, a number of mechanisms are available, such as RDF/XML, N-Triples [N-TRIPLES], and of course RDFa. Most discussions of RDF make use of the N-Triple syntax to explain their ideas, since it's quite compact. The examples we have just seen are already using this syntax, and we'll continue to use it throughout this document, with a slight variation that long URIs can be abbreviated by using a URI mapping. This is indicated by removing the angle brackets from the URI, as follows:
<http://dbpedia.org/resource/Albert_Einstein> foaf:name "Albert Einstein" . <http://dbpedia.org/resource/Albert_Einstein> p:dateOfBirth "1879-03-14"^^xs:date . <http://dbpedia.org/resource/Albert_Einstein> p:birthPlace <http://dbpedia.org/resource/Germany>. <http://dbpedia.org/resource/Albert_Einstein> foaf:depiction <http://en.wikipedia.org/wiki/Image:Albert_Einstein_Head.jpg>.Note that this is merely a way to make examples more compact and the actual triples generated would use the full URIs.
When writing examples, you will often see the following URI:
<>This indicates the 'current document', i.e., the document being processed.
A collection of triples is called a graph.
For more information on the concepts described above, see [RDF-CONCEPTS]. RDFa additionally defines the following terms:
In order to allow for the compact expression of RDF statements, RDFa allows the contraction of all [URI reference]s into a form called a 'compact URI', or [CURIE]. Until recently QNames [QNames] have been the most common way to abbreviate URIs, but there is a well-known limitation that the syntax for QNames does not allow all possible [URI reference]s to be expressed. CURIEs have been specificially designed to look like QNames, but at the same time to get around their limitations.
Note that CURIEs are only used in the mark-up, and never appear in the generate [triple]s, which will always use [URI reference]s.
The following is a description of RDFa that uses RDF terminology, which may be useful to readers with an RDF background:
The aim of RDFa is to allow [RDF graph]s to be carried in XML documents of any type, although this specification deals only with RDFa in XHTML. 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 [URI reference] or a [blank node], the predicate is always an [URI reference], and the object of a statement can be an [URI reference], a [literal], or a [blank node].
In RDFa, a subject [URI reference] is generally indicated using the attribute about
, and predicates are represented using one of the attributes property
,
instanceof
, rel
, or rev
. Objects which are [URI reference]s are represented using the attributes href
, resource
or src
, whilst
objects that are [literal]s are represented either with the attribute content
(with an optional [datatype] expressed using the datatype
attribute), or the content of the
element in question.
This section is normative.
This section looks at a generic set of processing rules for creating a set of triples that represent the metadata present in an XHTML+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').
Parsing a document for RDFa triples is carried out by starting at the root element of the document, and visiting each of its child elements in turn, 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.
As processing continues, rules are applied which will either generate triples, or change the [evaluation context] information which will be used in subsequent processing. Some of the rules will be determined by the host language--in this case XHTML--and some of the rules will be part of RDFa.
Note that we don't 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 conformant, an RDFa processor needs to act as if at least the rules in this section are applied.
During processing, each rule is applied within an 'evaluation context'. Rules may further modify this evaluation context, or create triples that can be established by making use of this evaluation context. The context itself consists of the following pieces of information:
base
element. The important
thing is that it establishes a URL against which relative paths can be evaluated.base
, but it will usually change during the course of processing. 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
SAX-style processing model to extract the RDFa information. Note that if some approach other than the DOM traversal approach defined here is used, it is important to ensure that any meta
or link
elements processed in the head
of the document honour any occurrences of base
which may appear after those elements. (In other words, HTML
processing rules must still be applied, even if document processing takes place in a non-HTML environment such as a search indexer.)
At the beginning of processing, the [current evaluation context] is initialised as follows:
base
element, if present;Processing then begins with the root element, and all nodes in the tree are processed according to the following rules, depth-first:
xmlns
atttribute. 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 the URI is not processed in any way; in particular if it is a relative path it is not resolved against the [current base]. Authors are advised to follow best practice for
using namespaces, which includes not using relative paths. (See [xyz].)xml:lang
, or the HTML attribute lang
.@about
. Note that the final value of the [current resource] is an absolute IRI, which means that if @about
contains a relative path the value must be normalised against [base] in the [current evaluation context], using the algorithm defined in RFC 3986. The value can also be provided by a CURIE, and is
processed as defined in section curie. Note that since this attribute can take both URIs and CURIEs, the latter will have been expressed using the [safe CURIE] syntax.resource
. If there is no resource
attribute then the HTML src
attribute is used, and if that is not present, the HTML href
attribute is used. If
none of these are present then a unique identifier or [bnode] is created. Note that the final value of the [current object resource] is an absolute URI, which means that if any of these attributes
contain relative paths they must be normalised against [base] in the [current evaluation context], using the algorithm defined in RFC 3986. Note also that since @resource
can take both
URIs and CURIEs, the latter will have been expressed using the [safe CURIE] syntax.content
attribute is present, or the body of the [current element] contains only
text (i.e., there are no child elements), or the body of the [current element] does have child elements but the datatype
attribute has an empty value. Additionally, if
there is a value for [current language] then the value of the [plain literal] should include this language information, as described here:???. The actual literal is either the value of the
content
attribute (if present) or a string created by concatenating the inner content of each of the children in turn, of the [current element].datatype
attribute is present, and does not have an empty value. The actual literal is
either the value of the content
attribute (if present) or a string created by concatenating the inner content of each of the children in turn, of the [current element]. The
final string includes the datatype, as described here:???datatype
attribute is not present. The value of
the [XML literal] is a string created from the inner content of the [current element], i.e., not including the element itself.property
attribute. If present, the attribute must contain one or more [basic curies], each
of which is converted to an absolute URI using CURIE processing rules, and then used to generate a triple as follows:
@rel
and @rev
attributes.rel
attribute must contain one or more [basic curies], each of which is converted to an absolute URI using CURIE processing rules, and then used to generate a triple as
follows:
rev
attribute must contain one or more [basic curies], each of which is converted to an absolute URI using CURIE processing rules, and then used to generate a triple as
follows:
true
.true
then the [current resource] is set to the value of the [current object resource], and the [chaining] flag is set to false
.instanceof
attribute. If present, the attribute must contain one or more [basic
curies], each of which is converted to an absolute URI using CURIE processing rules, and then used to generate a triple as follows:
true
.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.
@instanceof situation
This section still needs the detail on whether@instanceof
should use @about
if it is present, or use the subject from chaining.NOTE: There isn't quite enough detail on chaining yet.
In the following examples, for brevity assume that the following namespace prefixes have been 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# |
p: | http://dbpedia.org/property/ |
rdfa: | http://www.w3.org/ns/rdfa/ |
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/ |
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 [current resource]. 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
When triples are created they will always be in relation to the [current resource]. When parsing begins the [current resource] will be the URI of the document being parsed, or a value as set by
base
.
Metadata about the document itself is usually placed in the head
:
<html> <head> <title>Jo's Friends and Family Blog</title> <link rel="foaf:primaryTopic" href="#bbq" /> <meta property="dc:creator" content="Jo" /> </head> <body> ... </body> </html>
although it is possible for the data to appear elsewhere:
<html> <head> <title>Jo's Blog</title> </head> <body> <h1><span property="dc:creator">Jo</span>'s blog</h1> <p> Welcome to my blog. </p> </body> </html>
The value of base
may change the initial value of [current resource]:
<html> <head> <title>Jo's Friends and Family Blog</title> <link rel="foaf:primaryTopic" href="#bbq" /> <meta property="dc:creator" content="Jo" /> <base href="http://www.example.org/jo/blog" /> </head> <body> ... </body> </html>
As processing progresses, any about
attributes will change the [current resource]. 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. In this mark-up the properties cal:summary
and cal:dtstart
becomes part of the 'event' object, rather than being to do with the
document:
<html> <head> <title>Jo's Friends and Family Blog</title> <link rel="foaf:primaryTopic" href="#bbq" /> <meta property="dc:creator" content="Jo" /> </head> <body> <p about="#bbq" instanceof="cal:Vevent"> I'm holding <span property="cal:summary"> one last summer Barbecue </span>, on <span property="cal:dtstart" content="20070916T1600-0500" datatype="xs:datetime"> September 16th at 4pm </span>. </p> </body> </html>
Other kinds of resources can be used to set the [current resource]:
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>.
<div about="photo1.jpg"> <span class="attribution-line">this photo was taken by <span property="dc:creator">Mark Birbeck</span> </span> </div>
Using xml:base
<p> <strong>NOTE:</strong> The formatting will probably be odd here, but what I'm trying to achieve is a kind of 'box' like you would have in a magazine. The intention is not to break the main flow, but to draw attention to this point. </p> <p>XHTML does not include the <code>xml:base</code> <a href="#ref_XMLBASE">[XMLBASE]</a> attribute by default. However, if it is part of the host language an RDFa parser must process it, and use its value to set [base].</p> <p>An example follows to show how <code>xml:base</code> affects the subject:</p> <example> <span xml:base="http://internet-apps.blogspot.com/"> <span about="" rel="dc:creator" href="http://www.blogger.com/profile/1109404" /> <span about="" property="dc:title" content="Internet Applications" /> </span> </example> <p>The triples generated would be as follows:</p> <example> <http://internet-apps.blogspot.com/> dc:creator <http://www.blogger.com/profile/1109404> . <http://internet-apps.blogspot.com/> dc:title "Internet Applications" . </example>
There are two types of object, [URI resources] and [literals].
A [literal] object can be set by using the property
attribute to express a [predicate], and then using either the attribute content
, or the inline text of the element
that property
is on.
A [URI resource] object can be set using one of the attributes rel
or rev
to express a [predicate], and then using one of href
, resource
or
src
.
An [object literal] will be generated when the property
attribute is present. The property
attribute provides the predicate, and the following sections describe how the
actual literal to be generated is determined.
The content
attribute can be used to indicate a [plain literal], as follows:
<meta about="http://internet-apps.blogspot.com/" property="dc:creator" content="Mark Birbeck" />
The [plain literal] can also be specified by using the content of the element:
<span about="http://internet-apps.blogspot.com/" property="dc:creator">Mark Birbeck</span>
Both of these examples give the following triple:
<http://internet-apps.blogspot.com/> dc:creator "Mark Birbeck" .
The value of the content
attribute is given precedence over any element content, so the following would give exactly the same triple:
<span about="http://internet-apps.blogspot.com/" property="dc:creator" content="Mark Birbeck">John Doe</span>
RDF allows [plain literal]s to have a language tag, as illustrated by the following example from [RDFTESTS-RDFMS-XMLLANG-TEST006]:
<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 the inline
text of the element:
<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:
<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>
[Literal]s can be given a data type using the datattype
attribute:
This can be represented in RDFa as follows:
<span property="cal:dtstart" content="20070916T1600-0500" datatype="xs:datetime"> September 16th at 4pm </span>.
<> cal:dtstart "20070916T1600-0500"^^xs:datetime .
EliasT comments
We need to explain which datatypes are allowed and emphasize "plaintext".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:
<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:
<> dc:title "E = mc&amp;lt;sup&amp;gt;2&amp;lt;/sup&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 has child elements. For example:
<h2 property="dc:title"> E = mc<sup>2</sup>: The Most Urgent Problem of Our Time </h2>
would generate the expected triple:
<> dc:title "E = mc<sup>2</sup>: The Most Urgent Problem of Our Time"^^rdf:XMLLiteral .
There will be situations where the extra mark-up is not actually part of the meaning of the literal, and can be ignored. In this situation an empty datatype
value can be used to
override the XML literal behaviour:
<p>You searched for <strong>Einstein</strong>:</p> <p about="http://dbpedia.org/resource/Albert_Einstein"> <span property="foaf:name" datatype="">Albert <strong>Einstein</strong></span> (March 14, 1879 – April 18, 1955) was a German-born theoretical physicist. </p>
Although the rendering of this page has highlighted the term the user searched for, setting datatype
to nothing ensures that the data is interpreted as a plain literal, giving the
following triples:
<http://dbpedia.org/resource/Albert_Einstein> foaf:name "Albert Einstein" .
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.Although the RDFa processing model requires visiting each node in the tree, if the processor meets an [XML literal] then it shouldn't process any further down the tree. This is to prevent triples being generated from mark-up that is not actually in the hierarchy. For example, we might want to set the title of something to some XHTML that includes RDFa:
<h2 property="dc:title"> Example 3: <span about="#bbq" instanceof="cal:Vevent">...</span> </h2>
Note that this does effectively mean that the presence of @property
inhibits any further processing, so authors should watch out for stray attributes, if they find they are getting
fewer triples than they had expected.
One or more [URI object]s is needed when the rel
or rev
attribute is present. Each attribute will cause triples to be generated when used with @href
,
@resource
or @src
.
The rel
and rev
attributes are essentially the inverse of each other; whilst rel
establishes a relationship between the [current resource] as subject, and
the [object resource] as the object, rev
does the exact opposite, and uses the [object resource] as the subject, and the [current resource] as the object.
@resource
to set the objectRDFa provides the resource
attribute as a way to set the object of statements. This is particularly useful when referring to resources that are not themselves navigable links:
<html> <head> <title>On Crime and Punishment</title> </head> <body> <blockquote 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>
href
attribute If no resource
attribute is present, then @href
can be used to set the object.
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 a URI:
<link about="mailto:daniel.brickley@bristol.ac.uk" rel="foaf:knows" href="mailto:libby.miller@bristol.ac.uk" />
It's also possible to use both rel
and rev
at the same time on an element. This is particularly useful when two things stand in two different relationships with each, for
example when a picture is taken by Mark, but that picture also depicts him:
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 two triples:
<photo1.jpg> dc:creator <http://www.blogger.com/profile/1109404> . <http://www.blogger.com/profile/1109404> foaf:img <photo1.jpg> .
@src
to set the objectThis 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 sets the subject for any nested triples which means that the same triples can be expressed using this, more compact, syntax:
<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>
When a triple predicate has been expressed using the rel
attribute, and no href
, src
, or resource
attribute exists on the same [RDFa element],
then the CURIE represented by this element is used as the object. This CURIE is affected by
the about
attribute, but if none is present the object is a bnode (bnodes are discussed further in Section bnode [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 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:
<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:
<> dc:creator _:div0 . _:div0 foaf:name "Ben Adida" . _:div0 foaf:mbox <mailto:ben@adida.net> .
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:
_: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:
<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:
<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>
RDFa in XHTML makes use of a number of XHTML attributes, as well as providing a few new ones:
about
,rel
,rev
,property
,href
,resource
,src
,datatype
,content
instanceof
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:
This document was authored and published by <a about="" rel="dc:creator dc:publisher" href="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:
<> dc:creator <http://example.org/~markb> . <> dc:publisher <http://example.org/~markb> .
The same applies to the rev
and property
attributes.
This section is normative.
Note that this syntax definition will ultimately be defined in an external document [CURIE].A basic CURIE is comprised of two components, a prefix and a reference. The prefix is separated from the reference by a colon (:
). It is possible to omit both the
prefix and the colon, or to omit just the prefix and leave the colon.
curie := [ [ prefix ] ':' ] reference
prefix := NCName
reference := irelative-ref (as defined in [IRI])
In some situations an attribute needs to be able to allow either a CURIE, or a normal IRI. Since it is difficult to distinguish between CURIEs and IRIs RDFa adds the notion of a [safe CURIE]. The syntax is simply to surround the CURIE with square brackets:
safe_curie := '[' curie ']'
NOTE: The following language-independent prose will be removed shortly, once we have finalised this.
CURIEs can be used in any language, including non-XML languages. Any language that wishes to make use of CURIEs must provide a context which consists of:
:p
);p
);_:p
).When CURIEs are used in RDFa in XHTML, the context is set as follows:
http://www.w3.org/1999/xhtml#
.clarify the 'no prefix' situation
The advantage of setting the 'no prefix' mapping to the XHTML namespace is that we no longer need a preprocessing step to handle XHTML link types, such as anext
. However, this does have
the effect of moving all other values into the XHTML namespace, such as openid.delegate
. An alternative is to prohibit unprefixed CURIEs, other than those defined by XHTML.A CURIE is a representation of a full IRI. This IRI is obtained by taking the currently in-scope mapping that is associated with prefix
, and concatenating it with
the reference
. The result MUST be a syntactically valid IRI [IRI].
This section is normative.
This section is informative.
2007-09-04: Migrated to XHTML 2 Working Group Publication System. Converted to a format that is consistent with REC-Track documents. Updated to reflect current processing model. Added normative definition of CURIEs. Started updating prose to be consistent with current task force agremeents. [ShaneMcCarron], [StevenPemberton], [MarkBirbeck]
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]
This section is informative.
At the time of publication, the participants in the W3C XHTML 2 Working Group were: