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 specification had been jointly developed by the RDFa Working Group and the
HTML Working Group. The document was
previously published via the HTML Working Group, but has since been transitioned
to the newly rechartered RDFa Working Group.
Changes in this version of the specification include:
- Made the Turtle output in Example #1 easier to understand for beginners. (editorial)
- Narrowed
link and meta elements validity in the body of a document to require the use of @property. (non-substantive)
- Changed the description of
@datetime processing to be easier to understand for implementers. (editorial)
- Noted that the
@datetime and rdf:HTML features are non-normative features from non-REC documents and that once those features are published in REC documents that a Proposed Edited Recommendation will be published for HTML+RDFa 1.1 making the features normative. (non-substantive)
- Documented the single formal objection to the specification (editorial).
This specification skipped the Candidate Recommendation phase due to
having four fully interoperable implementations available before entering
the Candidate Recommendation phase. The final
implementation report
considered by the Director has been made available to the public.
This specification is an extension to the HTML5 language. All normative
content in the HTML5 specification, unless specifically overridden by this
specification, is intended to be the basis for this specification.
Note
There are two features in this
specification, @datetime processing and rdf:HTML
literals, that are currently defined as non-normative features. The intent
is that these features will eventually become normative features once the
specification that describes the @datetime attribute [HTML5]
and the specification that defines rdf:HTML [RDF-CONCEPTS]
become W3C Recommendations.
Implementers should implement these features now;
a 2nd (or later) edition of this specification will clarify the long-term
stability of those features. Based on the discussion between the RDFa
Working Group, the HTML Working Group, and the RDF Working Group,
it is not expected that implementation changes will be necessary as
HTML5 and RDF 1.1 advance to Recommendation.
There has been a single
formal objection
filed on this specification, arguing that the use of prefixes is too complicated
for a Web technology. The RDFa WG
made changes
based on the commenters feedback.
The commenter did not respond to the changes made. Three requests for feedback
were made over the course of several months with no response from the
commenter. The RDFa WG does not believe the changes made would be enough
to address the commenters request, which asked that that prefixes should be
removed entirely or hard-coded.
A sample test harness is
available for software developers. This set of tests is not intended to be
exhaustive.
A community-maintained website contains more
information on further reading, developer tools, and software libraries
that can be used to extract RDFa data from web documents.
This document was published by the RDFa Working Group as a Proposed Recommendation.
This document is intended to become a W3C Recommendation.
The W3C Membership and other interested parties are invited
to review the document and send comments to
public-rdfa-wg@w3.org
(subscribe,
archives)
through 23 July 2013.
Advisory Committee Representatives should consult their
WBS questionnaires.
Note that substantive technical comments were expected during the Last Call review period that ended 28 February 2013.
Publication as a Proposed Recommendation 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.
The Proposed Recommendation review period ends on 23 July 2013.
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.
3. Extensions to RDFa Core 1.1
The RDFa Core 1.1 [RDFA-CORE] specification is the base document on
which this specification builds.
RDFa Core 1.1 specifies the attributes and syntax, in Section 5: Attributes and
Syntax, and processing model, in Section 7: Processing
Model, for extracting RDF from a web document. This section
specifies changes to the attributes and processing model defined in
RDFa Core 1.1 in order to support extracting RDF from HTML documents.
The requirements and rules, as specified in RDFa Core and further
extended in this document, apply to all HTML5 documents. An RDFa processor
operating on both HTML and XHTML documents, specifically on their
resulting DOMs or infosets, MUST apply these processing rules
for HTML4, HTML5 and XHTML5
serializations, DOMs and/or infosets.
3.1 Additional RDFa Processing Rules
Documents conforming to the rules in this specification are processed
according to [RDFA-CORE] with the following extensions:
- The default vocabulary URI is undefined.
- HTML+RDFa uses an additional initial context by default,
http://www.w3.org/2011/rdfa-context/html-rdfa-1.1, which must
be applied after the initial context for [RDFA-CORE]
(http://www.w3.org/2011/rdfa-context/rdfa-1.1). - The
base can be set
using the
base element. For XHTML5+RDFa 1.1 documents,
base can also be set using the @xml:base
attribute.
- The
current language
can be set using either the
@lang or @xml:lang
attributes. When the @lang attribute and the
@xml:lang attribute are specified on the same element, the
@xml:lang attribute takes precedence. When both
@lang and @xml:lang are specified on the same
element, they MUST have the same value.
- When determining which set of RDFa processing rules to use for documents
served with the
application/xhtml+xml media type, a conforming
RDFa processor MUST look at the value in the DOCTYPE declaration of the
document. If a DOCTYPE declaration exists, then the
processing rules are:
- XHTML1+RDFa 1.0 for a DOCTYPE of
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML+RDFa 1.0//EN" "http://www.w3.org/MarkUp/DTD/xhtml-rdfa-1.dtd">, or
- XHTML1+RDFa 1.1 for a DOCTYPE of
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML+RDFa 1.1//EN" "http://www.w3.org/MarkUp/DTD/xhtml-rdfa-2.dtd">, or
- XHTML5+RDFa 1.1 for all other values of DOCTYPE.
Documents served as application/xhtml+xml, that don't contain
a DOCTYPE declaration, and don't specify a @version attribute MUST be interpreted
as XHTML5+RDFa 1.1 documents.
- In
Section 7.5:
Sequence, processing step 3, if the
processor graph
feature is supported and if an
IRI mapping
overwrites a previously existing mapping in the
local list of IRI mappings
with a different value, the processor MUST generate an appropriate
rdfa:PrefixRedefinition
warning and place the associated triples into the
processor graph.
- In
Section 7.5:
Sequence, immediately after processing step 4, if the
@property attribute and the @rel and/or
@rev attribute exists on the same element, the non-CURIE and
non-URI @rel and @rev values are ignored. If, after
this, the value of @rel and/or @rev becomes empty,
then the processor MUST act as if the respective attribute is not present.
- In Section 7.5,
processing step 5, and
processing step 6, if no IRI is provided by a resource attribute
(e.g.,
@about, @href, @resource, or
@src), then first check to see if the element is the
head or body element. If it is, then set
new subject
to
parent object.
- In
Section 7.5: Sequence, processing step 11, the
HTML5
@datetime attribute
MUST be utilized when generating
the current property value, unless @content is also
present on the same element. Otherwise, if @datetime is
present, the current property value must be generated as
follows. The literal value is the value contained in the
@datetime attribute. If @datatype is
present, it is to be used as defined in the RDFa Core [RDFA-CORE]
processing rules. Otherwise, if the value of
@datetime lexically matches a valid
xsd:date, xsd:time, xsd:dateTime,
xsd:duration, xsd:gYear, or
xsd:gYearMonth a typed literal must be generated, with its
datatype set to the matching xsd datatype. Otherwise, a plain literal
MUST be generated, taking into account the
current language.
Implementers should note that the correct order of match testing should be:
xsd:duration, xsd:dateTime,
xsd:date, xsd:time,
xsd:gYearMonth, and xsd:gYear.
This feature is currently non-normative,
see the note on when it will become
normative.
- In
Section 7.5: Sequence, processing step 11, if the element is
time, and the element does not have @datetime
or @content attributes, the processor MUST act as if there
is a @datetime attribute containing exactly the elements
text value. This feature is currently non-normative,
see the note on when it will become
normative.
- In
Section 7.5:
Sequence, step 11, immediately after sub-step 2, if the
@datatype attribute is present and evaluates to
http://www.w3.org/1999/02/22-rdf-syntax-ns#HTML,
the value of the HTML Literal is a string
created by serializing all child nodes to text. This applies to all nodes
that are descendants of the current
element, not including the element itself. The HTML Literal is
given a
datatype of http://www.w3.org/1999/02/22-rdf-syntax-ns#HTML
as defined in
Section 5.2: The rdf:HTML Datatype
of [RDF-CONCEPTS]. This feature is currently non-normative,
see the note on when it will become
normative.
- Once the
output graph
is generated following the processing steps defined in
Section 7.5: Sequence of
the RDFa Core 1.1 specification [RDFA-CORE], and the steps in this
section, perform the operations defined in
Implementing Property Copying.
The @version attribute is not supported in HTML5 and is
non-conforming. However, if an HTML+RDFa document contains the
@version attribute on the html element, a conforming
RDFa processor MUST examine the value of this attribute. If the value matches
that of a defined version of RDFa, then the processing rules for that version
MUST be used. If the value does not match a defined version, or there is no
@version attribute, then the processing rules for the most recent
version of RDFa 1.1 MUST be used.
3.3 Specifying the Language for a Literal
According to RDFa Core 1.1 the
current language
MAY be specified by the host language. In order to conform
to this specification, RDFa processors MUST use the mechanism described in
The lang and xml:lang attributes section of the [HTML5]
specification to determine the
language
of a node.
If the final encapsulating MIME type for an HTML fragment is not decided
on while editing, it is RECOMMENDED that the author
specify both @lang and @xml:lang where the value in
both attributes is exactly the same.
Note
The HTML5 specification takes the
Content-Language HTTP header into account when determining the
language of an element. Some RDFa processor implementations, like those
written in JavaScript, may not have
access to this header and will be non-conforming in the edge case where
the language is only specified in the Content-Language HTTP
header. In these instances, RDFa document authors are urged to
set the language in the document via the @lang
attribute on the html element in order to ensure
that the document is interpreted correctly across all RDFa processors.
3.4 Invalid XMLLiteral Values
When generating literals of type XMLLiteral, the processor MUST ensure
that the output XMLLiteral is a namespace well-formed XML fragment. A
namespace well-formed XML fragment has the following properties:
- The XML fragment, when placed inside of a single root element, MUST
validate as well-formed XML. The normative language that describes a
well-formed XML document is specified in Section 2.1 "Well-Formed
XML Documents" of the XML specification.
- The XML fragment, when placed inside of a single root element, MUST
retain all active namespace information. The currently active attributes
declared using
@xmlns and @xmlns: that are stored in the
RDFa processor's current
evaluation context
in the
IRI mappings
MUST be preserved in the generated XMLLiteral. The PREFIX value for
@xmlns:PREFIX MUST be entirely transformed into lower-case characters
when preserving the value in the XMLLiteral. All active namespaces declared
via @xmlns, @xmlns:, and @prefix
MUST be placed in each top-level element in the generated XMLLiteral,
taking care to not overwrite pre-existing namespace values.
An RDFa processor that transforms the XML fragment MUST use the
Coercing an HTML DOM into an infoset algorithm, as specified in the HTML5
specification, followed by the algorithm defined in the Serializing
XHTML Fragments section of the HTML5 specification. If an error or
exception occurs at any point during the transformation, the triple containing
the XMLLiteral MUST NOT be generated.
Transformation to a namespace well-formed XML fragment is required
because an application that consumes XMLLiteral data expects that data to
be a namespace well-formed XML fragment.
The transformation requirement does not apply to plain text input data that are
text-only, such as literals that contain a @datatype attribute
with an empty value (""), or input data that contain only
text nodes.
An example transformation demonstrating the preservation of namespace
values is provided below. The → symbol is used to denote that the line
is a continuation of the previous line and is included purely for the
purposes of readability:
Example 3: Namespace preservation markup
<p xmlns:ex="http://example.org/vocab#"
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">
Two rectangles (the example markup for them are stored in a triple):
<svg xmlns="http://www.w3.org/2000/svg"
property="ex:markup" datatype="rdf:XMLLiteral">
→<rect width="300" height="100" style="fill:rgb(0,0,255);stroke-width:1; stroke:rgb(0,0,0)"/>
→<rect width="50" height="50" style="fill:rgb(255,0,0);stroke-width:2;stroke:rgb(0,0,0)"/></svg>
</p>The markup above SHOULD produce the following triple, which preserves the
xmlns declaration in the markup by injecting the @xmlns attribute
in the rect elements:
Example 4: Namespace preservation triple
<>
<http://example.org/vocab#markup>
"""<rect xmlns="http://www.w3.org/2000/svg" width="300"
→height="100" style="fill:rgb(0,0,255);stroke-width:1; stroke:rgb(0,0,0)"/>
→<rect xmlns="http://www.w3.org/2000/svg" width="50"
→height="50" style="fill:rgb(255,0,0);stroke-width:2;
→stroke:rgb(0,0,0)"/>"""^^<http://www.w3.org/1999/02/22-rdf-syntax-ns#XMLLiteral> .Since the ex and rdf
namespaces are not used in either rect element, they are not
preserved in the XMLLiteral.
Similarly, compound document elements that reside in different
namespaces must have their namespace declarations preserved:
Example 5: Namespace preservation for compound document elements
<p xmlns:ex="http://example.org/vocab#"
xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
xmlns:fb="http://www.facebook.com/2008/fbml">
This is how you markup a user in FBML:
<span property="ex:markup" datatype="rdf:XMLLiteral">
→<span><fb:user uid="12345">The User</fb:user></span>
→</span>
</p>
The markup above SHOULD produce the following triple, which preserves the
fb namespace in the corresponding triple:
Example 6: Namespace element preservation triple
<>
<http://example.org/vocab#markup>
"""<span xmlns:fb="http://www.facebook.com/2008/fbml">
→<fb:user uid="12345"></fb:user>
→</span>"""^^<http://www.w3.org/1999/02/22-rdf-syntax-ns#XMLLiteral> .3.5 Property Copying
There are times when authors will find that they have many resources on a
page that share a common set of properties. For example, several
music events may have different performance times, but use
the same location, band, and ticket prices. In this particular case, it is
beneficial to have a short-hand notation to instruct the RDFa processor
to include the location, band, and ticket price information without having
to repeat all of the markup that expresses the data.
HTML+RDFa defines a property copying mechanism which allows
properties associated with a resource to be copied to another resource.
This mechanism can be activated by using the rdfa:copy
predicate.
The feature is illustrated in the following two examples:
Example 7: Events with duplicate properties
<div vocab="http://schema.org/">
<p typeof="MusicEvent">
<link property="image" href="Muse1.jpg"/>
<link property="image" href="Muse2.jpg"/>
<link property="image" href="Muse3.jpg"/>
<span property="name">Muse</span> at the United Center.
<time property="startDate" datetime="20130403">March 3rd 2013</time>,
<a property="location" href="#united">United Center, Chicago, Illinois</a>
...
</p>
<p typeof="MusicEvent">
<link property="image" href="Muse1.jpg"/>
<link property="image" href="Muse2.jpg"/>
<link property="image" href="Muse3.jpg"/>
<span property="name">Muse</span> at the Target Center.
<time property="startDate" datetime="20130703">March 7th 2013</time>,
<a property="location" href="#target">Target Center, Minneapolis, Minnesota</a>
...
</p>
</div>
In this case, two music events are defined with image,
name, startDate, and location properties. The
image and name values are identical for the two
events and are unnecessarily duplicated in the markup.
Using RDFa's property copying feature, a pattern can be
declared that expresses the common properties. This pattern can then be
copied into other resources within the document:
Example 8: Copying common properties
<div vocab="http://schema.org/">
<div resource="#muse" typeof="rdfa:Pattern">
<link property="image" href="Muse1.jpg"/>
<link property="image" href="Muse2.jpg"/>
<link property="image" href="Muse3.jpg"/>
<span property="name">Muse</span>
</div>
<p typeof="MusicEvent">
<link property="rdfa:copy" href="#muse"/>
Muse at the United Center.
<time property="startDate" datetime="20130403">March 3rd 2013</time>,
<a property="location" href="#united">United Center, Chicago, Illinois</a>
...
</p>
<p typeof="MusicEvent">
<link property="rdfa:copy" href="#muse"/>
Muse at the Target Center.
<time property="startDate" datetime="20130703">March 7th 2013</time>,
<a property="location" href="#target">Target Center, Minneapolis, Minnesota</a>
...
</p>
</div>In this case, the common properties for all of the events are expressed in
the first div. The common properties are copied into each
event resource via the rdfa:copy predicate. The output for the
previous two examples is the same:
Example 9: Turtle output of property copying example
@prefix schema: <http://schema.org/> .
[] a schema:MusicEvent;
schema:image <Muse1.jpg>, <Muse2.jpg>, <Muse3.jpg>;
schema:name "Muse";
schema:startDate "March 3rd 2013";
schema:location <#united> .
[] a schema:MusicEvent;
schema:image <Muse1.jpg>, <Muse2.jpg>, <Muse3.jpg>;
schema:name "Muse";
schema:startDate "March 7th 2013";
schema:location <#target> .
The copy process is iterative, so that resources may copy
other resources that copy other resources. For example:
Example 10: Resources may copy other resources that copy other resources
<div vocab="http://schema.org/">
<div typeof="Person">
<link property="rdfa:copy" href="#lennon"/>
<link property="rdfa:copy" href="#band"/>
</div>
<p resource="#lennon" typeof="rdfa:Pattern">
Name: <span property="name">John Lennon</span>
<p>
<div resource="#band" typeof="rdfa:Pattern">
<div property="band" typeof="MusicGroup">
<link property="rdfa:copy" href="#beatles"/>
</div>
</div>
<div resource="#beatles" typeof="rdfa:Pattern">
<p>Band: <span property="name">The Beatles</span></p>
<p>Size: <span property="size">4</span> players</p>
</div>
</div>In the example above, the properties from #lennon and
#band are copied into the first resource. Then the
properties from #beatles are copied into
#band. Subsequently, those properties are again copied into
the first resource yielding the following output:
Example 11: Iterative copying example output in Turtle
@prefix schema: <http://schema.org/> .
[ a schema:Person;
schema:name "John Lennon" ;
schema:band [
a schema:MusicGroup;
schema:name "The Beatles";
schema:size "4"
]
] .Similar to
Vocabulary Expansion
as defined in [RDFA-CORE], RDFa Property Copying operates on the
output graph
after document processing is complete.
3.5.1 Implementing Property Copying
Once the
output graph
is generated following the processing steps defined in
Section 7.5: Sequence of
the RDFa Core 1.1 specification [RDFA-CORE],
and the Extensions to the HTML5 Syntax
defined in this specification, processors MUST update the
output graph
using the following rules:
- Run the following rule for each
rdfa:copy statement
in the
output graph,
and for each new rdfa:copy statement added as a result of
property copying until no new triples are added to the
output graph:
| Rule name | If output graph contains | then add |
|---|
| pattern-copy |
?subject rdfa:copy ?target
?target rdf:type rdfa:Pattern
?target ?predicate ?object
|
?subject ?predicate ?object
|
- Finally, run this rule to remove utilized
rdfa:copy
statements and rdfa:Pattern resources from the
output graph:
| Rule name | If output graph contains | then remove |
|---|
| pattern-clean |
?subject rdfa:copy ?target
?target rdf:type rdfa:Pattern
?target ?predicate ?object
|
?subject rdfa:copy ?target
?subject rdf:type rdfa:Pattern
?target ?predicate ?object
|
Note
Implementers should be aware that a simplistic implementation of
the pattern-copy rule may lead to an infinite loop when
handling circular dependencies. A processor should cease
the pattern-copy rule when no unique triples are generated.
Note
Alternate rules may be used to update the output graph as
long as the end result is the same.
5. Backwards Compatibility
RDFa Core 1.1 deprecates the usage of @xmlns: in RDFa 1.1
documents. Web page authors SHOULD NOT use @xmlns: to express
prefix mappings in RDFa 1.1 documents. Web page authors SHOULD use
the @prefix attribute to specify prefix mappings.
However, there are times when XHTML+RDFa 1.0 documents are served by web
servers using the text/html MIME type. In these instances, the
HTML5 specification asserts that the document is processed according to the
non-XML mode HTML5 processing rules. In these particular cases, it is
important that the prefixes declared via @xmlns: are preserved
for the RDFa processors to ensure backwards-compatibility with RDFa 1.0
documents. The following sections elaborate upon the backwards compatibility
requirements for RDFa processor implementations.
5.1 @xmlns:-Prefixed Attributes
The RDFa Core 1.1 [RDFA-CORE] specification deprecates the
use of the @xmlns: mechanism to declare CURIE prefix mappings in
favor of the @prefix attribute. However, there are instances
where its use is unavoidable. For example, publishing legacy documents as HTML5 or
supporting older XHTML+RDFa 1.0 documents that rely on the @xmlns:
attribute.
CURIE prefix mappings specified using attributes prepended with
@xmlns: MUST be processed using the algorithm defined in
section 4.4.1:
Extracting URI Mappings from Infosets
for infoset-based processors, or section 4.5.1:
Extracting URI Mappings from DOMs
for DOM Level 2-based processors. For CURIE prefix mappings using the
@prefix attribute,
Section 7.5: Sequence, step 3
MUST be used to process namespace values.
Since CURIE prefix mappings have been specified using
@xmlns:, and since HTML attribute names are case-insensitive,
CURIE prefix names declared using the @xmlns:attribute-name
pattern xmlns:<PREFIX>="<URI>" SHOULD be specified
using only lower-case characters. For example, the text
"@xmlns:" and the text in "<PREFIX>" SHOULD
be lower-case only. This is to ensure that prefix mappings are interpreted
in the same way between HTML (case-insensitive attribute names) and XHTML
(case-sensitive attribute names) document types.
5.3 Preserving Namespaces via Coercion to Infoset
RDFa 1.0 documents may contain the @xmlns: pattern to
declare prefix mappings, it is important that namespace information that
is declared in non-XML mode HTML5 documents are mapped to an infoset
correctly. In order to ensure this mapping is performed correctly, the
"Coercing an HTML DOM into an infoset" rules defined in [HTML5]
must be extended to include the following rule:
If the XML API is namespace-aware, the tool must ensure that
([namespace
name], [local name],
[normalized
value]) namespace tuples are created when converting the non-XML mode
DOM into an infoset. Given a standard @xmlns: definition,
xmlns:foo="http://example.org/bar#", the [namespace name]
is http://www.w3.org/2000/xmlns/,
the [local name] is foo, and the
[normalized value] is http://example.org/bar#, thus the
namespace tuple would be (http://www.w3.org/2000/xmlns/,
foo, http://example.org/bar#).
For example, given the following input text:
Example 12
<div xmlns:com="https://w3id.org/commerce#">
The div element above, when coerced from an HTML DOM into
an infoset, should contain an attribute in the [namespace
attributes] list with a [namespace name] set to
"http://www.w3.org/2000/xmlns/", a [local name] set to
com, and a [normalized value] of
"https://w3id.org/commerce#".
5.4 Infoset-based Processors
While the intent of the RDFa processing instructions is to provide a
set of rules that are as language and toolchain independent as possible, for
the sake of clarity, detailed methods of extracting RDFa content from
processors operating on an XML Information Set are provided below.
5.4.2 Processing RDFa Attributes
There are a number of non-prefixed attributes that are associated with
RDFa Processing in HTML5. If an XML Information Set based RDFa processor is
used to process these attributes, the following algorithm should be used to
detect and extract the values of the attributes.
While processing Infoset Attribute Information Items in Element Information
Items as described in [RDFA-CORE],
Section 7.5: Sequence, Step #4 through Step #9:
- For each Attribute Information Item specific to RDFa in the infoset
[attributes]
list that has a [prefix] with
no value, extract and use the [normalized
value].
5.5 DOM Level 1 and Level 2-based Processors
Most DOM-aware RDFa processors are capable of accessing DOM Level 1
[DOM-LEVEL-1]
methods to process attributes on elements. To discover all
@xmlns:-specified CURIE prefix mappings, the
Node.attributes
NamedNodeMap can be iterated over. Each
Attr.name that
starts with the text string @xmlns: specifies a CURIE prefix
mapping. The value to be mapped is the string after the @xmlns:
substring in the Attr.name variable and the value to be mapped is
the value of the Attr.value variable.
The intent of the RDFa processing instructions are to provide a
set of rules that are as language and toolchain independent as possible. If
a developer chooses to not use the DOM1 environment mechanism outlined in
the previous paragraph, they may use the following DOM2 [DOM-LEVEL-2-CORE]
environment mechanism.
5.5.2 Processing RDFa Attributes
There are a number of non-prefixed attributes that are associated with
RDFa processing in HTML5. If an DOM2-based RDFa processor is used to
process these attributes, the following algorithm should be used to detect
and extract the values of the attributes.
While processing an element as described in [RDFA-CORE],
Section 5.5: Sequence,
Step #3 through Step #9:
- For each RDFa attribute in the [Node.attributes]
list that has a [namespace
prefix] that is null, extract and use [Node.nodeValue]
as the value.
Note
When extracting values from @href and
@src, web authors and developers should
note that certain values MAY be transformed if accessed via the DOM versus
a non-DOM processor. The rules for modification of URL values can be
found in the main HTML5 specification under
Section 2.6.2: Parsing URLs.
A. About this Document
A.1 History
This section is non-normative.
In early 2004, Mark Birbeck published a document named "RDF in XHTML"
via the XHTML2 Working Group wherein he laid
the groundwork for what would eventually become RDFa (The Resource
Description Framework in Attributes).
In 2006, the work was co-sponsored by the Semantic Web Deployment Working
Group, which began to formalize a technology to express semantic data in
XHTML. This technology was successfully developed and reached consensus at
the W3C, later published as an official W3C Recommendation. While HTML
provides a mechanism to express the structure of a document (title,
paragraphs, links), RDFa provides a mechanism to express the meaning in a
document (people, places, events).
The document, titled "RDF in XHTML: Syntax and Processing" [XHTML-RDFA],
defined a set of attributes and rules for
processing those attributes that resulted in the output of machine-readable
semantic data. While the document applied to XHTML, the attributes and
rules were always intended to operate across any tree-based structure
containing attributes on tree nodes (such as HTML4, SVG and ODF).
While RDFa was initially specified for use in XHTML, adoption by a
number of large organizations on the web spurred RDFa's use in non-XHTML
languages. Its use in HTML4, before an official specification was developed
for those languages, caused concern regarding document conformance.
Over the years, the members of the
RDFa Community had discussed the possibility
of applying
the same attributes and processing rules outlined in the XHTML+RDFa
specification to all HTML family documents. By design, the possibility of a
unified semantic data expression mechanism between all HTML and XHTML
family documents was squarely in the realm of possibility.
An RDFa Working Group was created in 2010 to address the issues concerning
multiple language implementations of RDFa. The XHTML+RDFa document was split
into a base specification, called RDFa Core 1.1 [RDFA-CORE], and thin
specifications that layer above RDFa Core 1.1. The XHTML+RDFa 1.1 specification
[XHTML-RDFA] is an example of such a thin specification. This
document, also a thin specification, is targeted at HTML4, HTML5 and
XHTML5.
This document describes the extensions to the RDFa Core 1.1
specification that permits the use of RDFa in all HTML family documents. By
using the attributes and processing rules described in the RDFa Core 1.1
specification and heeding the minor changes in this document, authors can
generate markup that produces the same semantic data output in
HTML4, HTML5 and XHTML5.
A.2 Change History
This section is non-normative.
2009-10-15: First version of the RDFa for HTML4, HTML5 and XHTML5.
2010-03-04: Updated HTML5 coercion to infoset rules, preservation of
namespaces in infoset and DOM2-based processors, clarifying how to
extract RDFa attributes via infoset, how to extract RDFa attributes via DOM2.
2010-05-02: Inheritance of basic processing rules from RDFa 1.1 [RDFA-CORE],
instead of XHTML+RDFa 1.0 [RDFA-SYNTAX], inclusion of the HTML Default
Vocabulary Terms, inclusion of a HTML 4.01 + RDFa 1.1 DTD for validation purposes.
2010-06-24: Inheritance of basic processing rules from RDFa 1.1 [RDFA-CORE],
instead of XHTML+RDFa 1.0 [RDFA-SYNTAX], inclusion of the HTML Default
Vocabulary Terms, added HTML 4.01 + RDFa 1.1 DTD for validation purposes,
added normative definition of @version attribute.
2010-10-19: Removal of @version attribute, migrated HTML Vocabulary Terms to
RDFa Profile document, added statement to send comments to the HTML WG bug tracker.
2011-01-11: Removed decentralized extensibility issue markers, added
DOM Level 1 prefix mapping extraction algorithm.
2011-04-05: Moved all xmlns: rules into a section titled Backwards
Compatibility and brought spec in-line with latest RDFa Core 1.1 spec.
2011-05-12: Generated Last Call document, no substantive changes.
2011-12-30: Addition of normative dependency for RDFa Lite 1.1.
Addition of rules to allow meta and
link elements in flow and phrasing content as long as they
contain at least one RDFa-specific attribute. Added support for
@datetime and value processing.
2012-03-10: Clarification of where each RDFa attribute is allowed to be
used. Feature at risk warning for HTML4+RDFa DTD-based validation.
2012-09-10: Publishing control of the HTML+RDFa document is handed over
from the HTML WG to the newly re-chartered RDFa WG. DTD-based validation is
removed from the specification.
2012-12-13: Addition of new HTML-specific processing rules for dealing with
XHTML5 vs. HTML5 documents, xml:base, HTML Literals, property and rel/rev on
the same element, and more types for the datetime attribute.
2012-12-27: Added Property Copying section and special processing for
head and body.
2013-01-19: Removed @value processing, added @content overriding @datetime
if present, cleanup and prep for Last Call publication in RDFa WG.
2013-06-06: Applied all Last Call comments and editorial fixes in
preparation for Proposed Recommendation phase.
A.3 Acknowledgments
This section is non-normative.
At the time of publication, the members of the RDFa Working Group were:
Ivan Herman (staff contact), Shane McCarron, Gregg Kellogg,
Niklas Lindström, Steven Pemberton, Manu Sporny (chair), Ted Thibodeau, and
Stéphane Corlosquet.
A great deal of thanks to everyone that provided feedback on the
specification (most of whom are listed below):
Adam Powell, Alex Milowski, Andy Seaborne, Arto Bendiken, Austin William, BAI Xi, Benjamin Adrian, Benjamin Nowack, Bjoern Hoehrmann, Christian Langanke, Christoph Lange, Cindy Lewis, Corey Mwamba, Crisfer Inmobiliaria, Dan Brickley, Daniel Friesen, Dave Beckett, David Wood, D. Grant, Dominik Tomaszuk, Dominique Hazael-Massieux, Doug Schepers, Dr. Olaf , Edward O'Connor, Faye Harris, Felix Sasaki, Gavin Carothers, Grant Robertson, Guus Schreiber, Harry Halpin, Michael Hausenblas, Henri Bergius, Henri Sivonen, Henry Story, Holger Knublauch, Ian Hickson, Irene Celino, Alexander Kroener, Knud Möller, Philip Jägenstedt, Reto Bachmann-Gmür, Ivan Mikhailov, James Leigh, Jeff Sonstein, Jeni Tennison, Jens Haupert, Jochen Rau, John Breslin, John Cowan, John O'Donovan, Jonathan Rees, Julian Reschke, KANZAKI Masahide, Kingsley Idehen, Knud Hinnerk, Landong Zuo, Leif Halvard Silli, Liam R., Lin Clark, Maciej Stachowiak, Mark Nottingham, Markus Gylling, Martin Hepp, Martin McEvoy, Matthias Tylkowski, Darin McBeath, Melvin Carvalho, Michael Chan, Michael Hausenblas, Michael Steidl, Michael™ Smith, Mischa Tuffield, Misha Wolf, Nathan Rixham, Nathan Yergler, Nicholas Stimpson, Noah Mendelsohn, Paul Cotton, Paul Sparrow, Pete Cordell, Peter Frederick, Peter Mika, Peter Occil, Phil Archer, Reece Dunn, Richard Cyganiak, Robert Leif, Robert Weir, Ramanathan V. Guha, Sami Korhonen, Sam Ruby, Sandro Hawke, Sebastian Germesin, Sebastian Heath, Shelley Powers, Simon Grant, Simon Reinhardt, Stefan Schumacher, Tab Atkins Jr., Thomas Adamich, Thomas Baker, Thomas Roessler, Thomas Steiner, Tim Berners-Lee, Toby Inkster, Tom Adamich, Tantek Çelik, Ville Skyttä, Wayne Smith, and Will Clark