literal
, specifically a basic string with no
type or language information:
Example 18


"Albert Einstein"
A plain literal can also be given a language tag, to capture plain
text in a natural language. For example, Einstein's birthplace has
different names in English and German:
Example 19

rdfs:label "German Empire"@en;
rdfs:label "Deutsches Kaiserreich"@de .
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 an IRI to the end of a
plain literal
and this IRI indicates the literal's datatype. This IRI is usually
based on datatypes defined in the XML Schema Datatypes specification
XMLSCHEMA11-2
]. The following syntax would be used to unambiguously
express Einstein's date of birth as a literal of type
Example 20


"1879-03-14"
^^
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 IRIs, 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 IRIs to be abbreviated by using an IRI mapping, which can
be used to express a compact IRI expression as follows:
Example 21
@prefix dbp: .
@prefix foaf: .

foaf:name
"Albert Einstein" .

dbp:birthPlace
.
Here 'dbp:' has been mapped to the IRI for DBPedia and 'foaf:' has
been mapped to the IRI for the 'Friend of a Friend' vocabulary.
Any IRI 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:
Example 22
@prefix dbp: .
@prefix dbr: .
@prefix foaf: .
@prefix rdfs: .
@prefix xsd: .
dbr:Albert_Einstein
foaf:name "Albert Einstein";
dbp:birthPlace dbr:German_Empire;
dbp:dateOfBirth "1879-03-14"^^xsd:date;
foaf:depiction .
dbr:German_Empire
rdfs:label
"German Empire"@en;
rdfs:label
"Deutsches Kaiserreich"@de .
When writing examples, you will often see the following IRI in the
Turtle representation:
Example 23
<>
This indicates the 'current document', i.e., the document being
processed. In the end there will always be a full IRI 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 IRIs.
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
output

graph
by an RDFa Processor. For more information on graphs
and other RDF concepts, see [
RDF-SYNTAX-GRAMMAR
].
3.8
Compact URI Expressions
In order to allow for the compact expression of RDF statements, RDFa
allows the contraction of most
IRI reference
s into a
form called a 'compact URI expression', or
CURIE
. A
detailed discussion of this mechanism is in the section
CURIE

and IRI Processing
Note that CURIEs are only used in the markup and Turtle examples, and
will never appear in the generated
triple
s, which are
defined by RDF to use
IRI reference
s.
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:
An
RDF graph
comprises
node
s linked by relationships. The aim of RDFa is to allow a single
RDF graph
to be carried in various types of document markup. The basic unit
of an
RDF graph
is a
triple
, in which a
subject
node
is linked to an object
node
via a
predicate
. The
subject
node
is always either a
IRI reference
or a
blank
node (or bnode)
, the
predicate
is
always
IRI reference
, and the object of a statement can be a
IRI reference
, a
literal
, or a
bnode
In RDFa, a subject
IRI reference
is generally indicated
using
@about
and predicates are represented using one of
@property
@rel
, or
@rev
Objects which are
IRI reference
s are represented using
@resource
@src
, or
@href
, whilst objects that are
literal
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
MAY
MUST
MUST NOT
RECOMMENDED
SHOULD
, and
SHOULD NOT
are
to be interpreted as described in [
RFC2119
].
4.1
RDFa Processor Conformance
This specification uses the term
output
graph
to mean all of the triples asserted by a document
according to the
Processing Model
section. 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.
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
output graph
and the
processor graph
are separate graphs and
MUST NOT
be stored in the same graph by the
RDFa Processor. However, processors may permit the two graphs to be
retrieved together; see
Section 7.6.1
for details.
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
output graph
(Whether these additional triples are made available in one or more
additional
RDF graph
s is implementation-specific, and
therefore not defined here.)
A conforming RDFa Processor
MUST
preserve white space in both
plain

literal
s 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., [
XMLSCHEMA11-1
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 Host Language
MAY
specify additional
announcement mechanisms.
Note
A conforming RDFa Processor
MAY
use additional
mechanisms (e.g., the DOCTYPE, a file extension, the root element, an overriding
user-defined parameter) 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 required attributes defined in this specification
MUST
be included in
the content model of the Host Language.
Note
For the avoidance of doubt, there is no requirement that attributes
such as
@href
and
@src
are used in a
conforming Host Language. Nor is there any requirement that all
required attributes are incorporated into the content model of
all elements. The working group recommends that Host Language designers
ensure that the required attributes are incorporated into the content
model of elements that are commonly used throughout 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:

).

When a Host Language does not use the attributes in 'no namespace',
they
MUST
be referenced via the XHTML Namespace (
).
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
specify an
initial context
(e.g., IRI mappings and/or a definition of terms or a default
vocabulary IRI). Such an
initial context
SHOULD
be
defined using the conventions defined in
RDFa

Initial Contexts
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
or
application/xml
. Such documents must meet all of the
following criteria:
The document
MUST
be well-formed as defined in [
XML10-4e
].
The document
SHOULD
use the attributes defined in this
specification in 'no namespace' (e.g., when the attributes are used on
elements they are used with no qualifying
prefix:

).
Note
It is possible that an XML grammar will have native attributes that
conflict with attributes in this specification. This could result in an RDFa
processor generating unexpected triples.
When an RDFa Processor processes an XML+RDFa document, it does so via
the following
initial context
There are default terms (e.g.,
describedby
license
, and
role
), defined
in
There are default prefix mappings (e.g.,
dc
),
defined in
There is no default vocabulary IRI.
The
base
can be set using the
@xml:base
attribute as defined in [
XML10-4e
].
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
SafeCURIEorCURIEorIRI
, used for stating what the
data is about (a 'subject' in RDF terminology);
content
CDATA
string, for supplying machine-readable content
for a literal (a 'literal object', in RDF terminology);
datatype
TERMorCURIEorAbsIRI
representing a datatype, to
express the datatype of a literal;
href
(optional)
a traditionally navigable
IRI
for expressing the
partner resource of a relationship (a 'resource object', in RDF
terminology);
inlist
An attribute used to indicate that the object
associated with a
rel
or
property
attribute on the same element is to be added to the list for that
predicate. The value of this attribute
MUST
be ignored.
Presence of this attribute causes a list to be created if it does not already exist.
prefix
a white space separated list of prefix-name IRI pairs of the form
NCName
':' ' '+ xsd:anyURI
property
a white space separated list of
TERMorCURIEorAbsIRIs
used for expressing relationships between a subject and either a resource
object if given or some literal
text (also a 'predicate');
rel
a white space separated list of
TERMorCURIEorAbsIRIs
used for expressing relationships between two resources ('predicates'
in RDF terminology);
resource
SafeCURIEorCURIEorIRI
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 of
TERMorCURIEorAbsIRIs
used for expressing reverse relationships between two resources (also
'predicates');
src
(optional)
an
IRI
for expressing the partner resource of a
relationship when the resource is embedded (also a 'resource object');
typeof
a white space separated list of
TERMorCURIEorAbsIRIs
that indicate the RDF type(s) to associate with a subject;
vocab
an
IRI
that defines the mapping to use when a
TERM
is referenced in an attribute value. See
General
Use of Terms in Attributes
and the
section
on Vocabulary Expansion
Note
In all cases it is possible for these attributes to be used with
no value (e.g.,
@datatype
="") or with a value that evaluates to
no value after evaluation using the rules for
CURIE and IRI Processing
(e.g.,
@datatype
="[noprefix:foobar]").
5.1
Roles of attributes
The RDFa attributes play different roles in a semantically rich document.
Briefly, those roles are:
Syntax attributes:
@prefix
@vocab
Subject attributes:
@about
Predicate attributes:
@property
@rel
@rev
Resource attributes:
@resource
@href
@src
Literal attributes:
@datatype
@content
@xml:lang
or
@lang
Macro attributes:
@typeof
@inlist
5.2
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.
Note
This definition is consistent with the definition found
in [
XML10
].
6.
CURIE Syntax Definition
Note
The working group is currently examining the productions
for CURIE below in light of recent comments received from the RDF
Working Group and members of the RDFa Working
Group. It is possible that there will be minor changes to the production
rules below in the near future, and that these changes will be
backward
incompatible
. However, any such incompatibility will be
limited to edge cases.
The key component of RDF is the IRI, but these are usually long and
unwieldy. RDFa therefore supports a mechanism by which IRIs can be
abbreviated, called 'compact URI expressions' or simply,
CURIEs
When expanded, the resulting IRI
MUST
be a syntactically valid IRI
RFC3987
]. For a more detailed explanation see
CURIE

and IRI Processing
. The
lexical space
of a CURIE is as
defined in
curie
below. The
value space
is the set of IRIs.
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
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.
Note
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
::= ( ipath-absolute / ipath-rootless / ipath-empty )
[ "?" iquery ] [ "#" ifragment ] (as defined in [[!RFC3987]])
curie ::= [ [ prefix ] ':' ] reference
safe_curie ::= '[' [ [ prefix ] ':' ] reference ']'
Note
The production
safe_curie
is not required,
even in situations where an attribute value is permitted to be a CURIE
or an IRI: An IRI 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.
Note
It is possible to define a CURIE prefix mapping in such a way that
it would overshadow a defined IRI scheme. For example, a document could map the prefix
'mailto' to 'http://www.example.com/addresses/'. Then a
@resource
that
contained 'mailto:user@example.com' might create a triple with the object
'http://www.example.com/addresses/user@example.com'. Moreover, it is possible
though unlikely, that schemes will be introduced in the future that will conflict
with prefix mappings defined in a document (e.g., the newly proposed 'widget'
scheme [
WIDGETS-URI
]). In neither case would this RDFa overshadowing of the
underlying scheme alter the way other consumers of the IRI treat that IRI. It
could, however, mean that the document author's intended use of the CURIE is
mis-interpreted by another consumer as an IRI. The working group considers this
risk to be minimal.
In normal evaluation of CURIEs the following context information would
need to be provided:
a set of mappings from prefixes to IRIs;
a mapping to use with the default prefix (for example,
:p
);
a mapping to use when there is no prefix (for example,
);
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 IRIs
is
provided by the current in-scope prefix declarations of the
current

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
bnode
s,

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 IRI. The rules for determining
that IRI are:
If a CURIE consists of an empty
prefix
and a
reference
the IRI 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 a
reference
, and if there is an in-scope mapping for
prefix
(when compared case-insensitively), then the IRI is created by using
that mapping, and concatenating it with the
reference
Finally, if there is no in-scope mapping for
prefix
then the value is not a CURIE.
Note
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 [
XMLSCHEMA11-2
]. 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 IRIs 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 an IRI
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 IRIs 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 IRIs, and any IRI can be represented by such an
expansion. QNames are treated as value pairs, but even if those
pairs are combined into a string, only a subset of IRIs 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.
Note
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 the
evaluation context
information that will then be used when processing descendant
elements.
Note
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 single
RDF graph
produced. As described in the
RDFa Processor
Conformance
section, any additional triples generated
MUST NOT
appear in the
output graph
. They
MAY
be included in
the
processor 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 IRI of the
document being processed, but it could be some other IRI, set by
some other mechanism, such as the (X)HTML
base
element. The important thing is that it establishes an IRI 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 member 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.
A list of current, in-scope
IRI

mappings
A list of
incomplete triple
s. 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 called
chaining
).
list mapping
that associates IRIs with lists.
The
language
. Note that there is no default
language.
The
term mappings
, a list of terms and their
associated IRIs. This specification does not define an initial list.
Host Languages
MAY
define an initial list.
The
default vocabulary
, a value to use as the prefix
IRI when a
term
unknown to the RDFa
Processor is used. This specification does not
define an initial setting for the default vocabulary. Host Languages
MAY
define an initial setting.
During the course of processing, new
evaluation context
are created which are passed to each child element. The initial rules
described below will determine the values of the items in the context.
Then the core 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
IRI mapping
s, called the
local list of IRI mappings
An initially empty
list of incomplete triples
called the
local list of incomplete triples
An initially empty
language
value.
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.
new subject
value, which once calculated will set
the
parent subject
in an
evaluation
context
, as well as being used to complete any
incomplete

triple
s, as described in the next section.
A value for the
current
property value
, the literal to use when creating triples
that have a literal object, or IRI-s in the absence of
@rel
or
@rev
A value for the
current
object resource
, the resource to use when creating triples
that have a resource object.
A value for the
typed resource
the source for creating
rdf:type
relationships to
types specified in
@typeof
The
local term mappings
, a list of terms and their
associated IRIs.
The
local list mapping
, mapping IRIs to lists
local default vocabulary
, an IRI 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:
Example 24

triple
from the residence part, and then use any number of
further subjects to 'complete' that triple, as follows:
Example 26

dbp-owl:residence .

dbp-owl:residence .
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:
Example 28

rel="foaf:depiction"

When such minimal markup is used, any of the resource-related
attributes could act as a subject or an object in the chaining:
Example 29
about="http://dbpedia.org/resource/Albert_Einstein"
rel="dbp-owl:residence"
about="http://dbpedia.org/resource/German_Empire"
>
about="http://dbpedia.org/resource/Switzerland"
>


Note that, as noted above, in many situations the
@property
and
@rel
are interchangeable. This is
not
true
for chaining. Taking the first example, if that example was used as
follows:
Example 30

Albert Einstein
1879-03-14
property="dbp:birthPlace"
resource="http://dbpedia.org/resource/German_Empire">
the German Empire

The subject for 'the German Empire' would remain Albert Einstein (and
that would, of course, be an error). This is the main difference
between
@property
and
@rel
: the latter
induces chaining, whereas the former, usually, does not.
7.4
CURIE and IRI Processing
Since RDFa is ultimately a means for transporting RDF, a key concept
is the
resource
and its manifestation as an IRI. RDF deals
with complete IRIs (not relative paths); when converting RDFa to
triples, any relative IRIs
MUST
be resolved relative to the base IRI,
using the algorithm defined in section 6.5 of RFC 3987 [
RFC3987
],
Reference

Resolution
. The values of
RDFa attributes
that refer to IRIs use three different datatypes:
IRI
SafeCURIEorCURIEorIRI
, or
TERMorCURIEorAbsIRI
All these attributes are mapped, after processing, to IRIs. The
handling of these attributes is as follows:
IRI
The content is an IRI, and is used as such.
SafeCURIEorCURIEorIRI
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 IRI is used; otherwise, the value is
processed as an IRI.
Note
A consequence of this is that when the value of an attribute of this
datatype is the empty string (e.g.,
@about
=""), that value resolves to an
IRI. An IRI of "" is a relative IRI that is interpreted as being the same as the
base
In other words, a value of "" will usually resolve to the IRI of the current document.
Note
A related consequence of this is that when the value of an attribute of this datatype is an empty SafeCURIE (e.g.,
@about
="[]"), that value does not result in an IRI and therefore the value is ignored.
TERMorCURIEorAbsIRI
If the value is a
term
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 IRI is used.
If the value is an absolute IRI, 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 IRI for Albert Einstein on DBPedia is:
Example 31
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 IRI.
In RDFa these mappings are expressed using
@prefix
Example 32
prefix="db: http://dbpedia.org/"
...

Once the prefix has been established, an author can then use it to
shorten an IRI as follows:
Example 33

...

The author is free to split the IRI at any point.
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:
Example 34
prefix="dbr: http://dbpedia.org/resource/"

...

...

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 IRI obtained from a CURIE is relative. However,
since all IRIs 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 IRIs expressed by the following two
CURIEs are different, despite the common prefix, because the prefix
mappings are locally scoped:
Example 35
prefix="dbr: http://dbpedia.org/resource/"

...

prefix="dbr: http://someotherdb.org/resource/"

...

Note
In general it is a bad practice to redefine prefix
mappings within a document. In particular, while it is permitted, mapping a
prefix to different values at different places within a document could lead to
confusion. The working group recommends that document authors use the same
prefix to map to the same vocabulary throughout a document. Many vocabularies
have recommended prefix names. The working group recommends that these names
are used whenever possible.
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:
An attribute may allow one or more values that are a mixture of
TERMs, CURIEs, and absolute IRIs.
An attribute may allow one or more values that are a mixture of
CURIEs and IRIs. In this case any value that is not a CURIE, as
outlined in section
CURIE Syntax Definition
will be processed as an IRI.
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.
Note
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 IRI by this specification.
An example of an attribute that can contain a CURIEorIRI is
@about
To express an IRI directly, an author might do this:
Example 36
about="http://dbpedia.org/resource/Albert_Einstein"
...

whilst to express the IRI above as a CURIE an author would do this:
Example 37
about="dbr:Albert_Einstein"
...

The author could also use a safe CURIE, as follows:
Example 38
about="[dbr:Albert_Einstein]"
...

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
TERM
s, and the CURIE
has no prefix separator.
Example 39
about="[Albert_Einstein]"
...

However, this markup
would
set a subject, since it is not
a CURIE, but a valid relative IRI:
Example 40
about="Albert_Einstein"
...

Note that several RDFa attributes are able to also take
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 ::=
NCNameStartChar
termChar
termChar ::= (
NameChar
- ':' ) | '/'
Note
For the avoidance of doubt, this production
means a 'term' in RDFa is an XML
NCName
that also permits
slash as a non-leading character.
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 an IRI using the following logic:
If there is a
local default vocabulary
the IRI is
obtained by concatenating that value and the
term
Otherwise, 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 IRI.
Otherwise, the
term
has no associated IRI and
MUST
be ignored.
Note
local default vocabulary
can be defined by the
Host Language as part of the
initial context
, and can be overridden on
the current element and its children using
@vocab
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
datatype
SafeCURIEorCURIEorIRI
- allowing a
SafeCURIE, a CURIE, or an IRI.
@href
and
@src
are as defined in the
Host Language (e.g., XHTML), and support only an IRI.
@vocab
supports an IRI.
@datatype
supports the datatype
TERMorCURIEorAbsIRI
- allowing a single Term, CURIE, or Absolute IRI.
@property
@typeof
@rel
and
@rev
support the datatype
TERMorCURIEorAbsIRIs
- allowing one or more Terms, CURIEs, or Absolute IRIs.
Any value that matches a defined term
MUST
be expanded into a
reference to the corresponding IRI. For example in
the following examples:
Example 41


would each generate the following triple:
Example 42
<> .
7.4.5
Referencing Blank Nodes
In RDFa, it is possible to establish relationships using various
types of resource references, including
bnode
s. 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 (e.g., 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:
Example 43
about="_:john"
rel="foaf:mbox"
href="mailto:john@example.org" />
about="_:sue"
rel="foaf:mbox"
href="mailto:sue@example.org" />
about="_:john"
rel="foaf:knows"
resource="_:sue" />
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:
Example 44
_:john foaf:mbox .
_:sue foaf:mbox .
_:john foaf:knows _:sue .
Note
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,
and
base
will affect the interpretation of IRIs in
meta
or
link
elements even if those elements are before the
base
element in the stream).
Note
In this section the term 'resource' is used to mean '
IRI
or
bnode
'. It is possible that this term will be replaced with
some other, more formal term after consulting with other groups. Changing this
term will in no way change this processing sequence.
At the beginning of processing, an initial
evaluation

context
is created, as follows:
the
base
is set to the IRI of the document (or
another value specified in a language specific manner such as the
HTML
base
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
list mapping
is empty;
the
language
is set to null.
the
list of IRI mappings
is empty (or a list defined
in the
initial context
of the Host
Language).
the
term mappings
is set to null (or a list defined in the
initial context
of the Host
Language).
the
default vocabulary
is set to null (or an IRI defined in
the
initial context
of the Host
Language).
Processing begins by applying the processing rules below to the
document object, in the context of this initial
evaluation
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.
Note
This specification defines processing rules for optional
attributes that may not be present in all Host Languages (e.g.,
@href
).
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:
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;
typed resource
is set to null;
the
local list of IRI mappings
is set to the
list of IRI mappings from the
evaluation context
the
local list of incomplete triples
is set to
null;
the
list mapping
is set to (a reference of) the
list mapping from the
evaluation context
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 an
evaluation 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.
Note that the
local term mappings
is always reset to a global
value, provided by the
initial context
Future versions of this specification may introduce
a mechanism whereby the
local term mappings
can be set dynamically, in which case the
local term mappings
would inherit from the parent's values.
Next the
current element
is examined for any change to the
default vocabulary
via
@vocab
. If
@vocab
is present and contains
a value, the
local default vocabulary
is updated
according to the section on
CURIE and IRI Processing
If the value is empty, then the
local
default vocabulary
MUST
be reset to the Host Language
defined default (if any).
The value of
@vocab
is used
to generate a triple as follows:
subject
base
predicate
object
value from
@vocab
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 setting the
local
default vocabulary
to null.
Next, the
current element
is
examined for
IRI mapping
s and these are added to the
local

list of IRI mappings
. Note that an
IRI 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 — an
IRI. (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 any
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 IRI 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 IRI.
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
).
If the
current element
contains
no
@rel
or
@rev
attribute, then the next
step is to establish a value for
new subject
. This
step has two possible alternatives.
If the
current element
contains the
@property
attribute, but does
not
contain either the
@content
or
@datatype
attributes, then
new subject
is set to
the resource obtained from the first match from the following rule:
by using the resource from
@about
, if present,
obtained according to the section on
CURIE

and IRI Processing
otherwise
, 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;
otherwise
, if
parent object
is
present,
new subject
is set to the value of
parent object
If
@typeof
is present then
typed resource
is set to the resource obtained from the first match from the following rules:
by using the resource from
@about
, if present,
obtained according to the section on
CURIE and IRI Processing
otherwise
, 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 previous
rule;
otherwise
by using the resource from
@resource
, if present,
obtained according to the section on
CURIE

and IRI Processing
otherwise
, by using the
IRI
from
@href
if present, obtained according to the section on
CURIE

and IRI Processing
otherwise
, by using the IRI from
@src
if present, obtained according to the section on
CURIE

and IRI Processing
otherwise
, the value of
typed resource
is
set to a newly created
bnode
The value of the
current object resource
is then set to the value of
typed resource
otherwise
If the element contains an
@about
@href
@src
, or
@resource
attribute,
new subject
is set to the resource obtained as follows:
by using the resource from
@about
, if present,
obtained according to the section on
CURIE
and IRI Processing
otherwise
, by using the resource from
@resource
, if present,
obtained according to the section on
CURIE

and IRI Processing
otherwise
, by using the
IRI
from
@href
if present, obtained according to the section on
CURIE

and IRI Processing
otherwise
, by using the IRI from
@src
if present, obtained according to the section on
CURIE

and IRI Processing
otherwise
, if no resource 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;
otherwise
, 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 of
parent object
. Additionally, if
@property
is
not
present then the
skip element
flag is set to 'true'.
Finally, if
@typeof
is present, set the
typed resource
to the value of
new subject
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
resource obtained from the first match from the following rules:
by using the resource from
@about
, if present,
obtained according to the section on
CURIE

and IRI Processing
if the
@typeof
attribute is present, set
typed

resource
to
new subject
If no resource 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;
otherwise
, if
parent object
is
present,
new subject
is set to that.
Then the
current object resource
is set to the
resource obtained from the first match from the following rules:
by using the resource from
@resource
, if present,
obtained according to the section on
CURIE

and IRI Processing
otherwise
, by using the
IRI
from
@href
if present, obtained according to the section on
CURIE

and IRI Processing
otherwise
, by using the
IRI
from
@src
if present, obtained according to the section on
CURIE

and IRI Processing
otherwise
, if
@typeof
is present and
@about
is not, use a newly created
bnode
If
@typeof
is present and
@about
is
not, set
typed resource
to
current object
resource
Note that final value of the
current object resource
will either be null (from initialization) or a full IRI or
bnode
If in any of the previous steps a
typed resource
was
set to a non-null value, it is now used to provide a subject for
type values;
One or more 'types' for the
typed
resource
can be set by using
@typeof
. If
present, the attribute may contain one or more IRIs, obtained
according to the section on
CURIE
and IRI Processing
, each of which is used to generate a
triple as follows:
subject
typed resource
predicate
object
current full IRI of 'type' from
typed resource
If in any of the previous steps a
new

subject
was set to a non-null value
different
from the
parent object
The
list mapping
taken from
the
evaluation context
is set to a new, empty
mapping.
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 and add entries to the
local list mapping
If the element contains
both
the
@inlist
and the
@rel
attributes the
@rel
may contain one or more resources, obtained according to the section on
CURIE and IRI Processing
each of
which is used to add an entry to the
list mapping
as
follows:
if the
local list mapping
does not contain a
list associated with the IRI, instantiate a new list and add
to local list mappings
add the
current object resource
to the list
associated with the resource in the
local list mapping
Predicates for the
current object
resource
can be set by using one or both of the
@rel
and the
@rev
attributes but, in case of the
@rel
attribute, only if the
@inlist
is
not
present:
If present,
@rel
may contain one or more resources,
obtained according to the section on
CURIE

and IRI Processing
each of which is used to generate a
triple as follows:
subject
new subject
predicate
full IRI
object
current object resource
If present,
@rev
may contain one or more resources,
obtained according to the section on
CURIE

and IRI Processing
each of which is used to generate a
triple as follows:
subject
current object resource
predicate
full IRI
object
new subject
If however
current object
resource
was set to null, but there are predicates present,
then they must be stored as
incomplete triple
s,
pending the discovery of a subject that can be used as the object.
Also,
current object resource
should be set to a newly
created
bnode
(so that the incomplete triples have a
subject to connect to if they are ultimately turned into triples);
Predicates for
incomplete triple
can be set by using one or both of the
@rel
and
@rev
attributes:
If present,
@rel
must contain one or more
resources, obtained according to the section on
CURIE

and IRI Processing
each of which is added to the
local

list of incomplete triples
as follows:
If the element contains the
@inlist
attribute, then
if the
local list mapping
does not
contain a list associated with the IRI, instantiate a
new list and add to local list mappings.
Add:
list
list from
local list mapping
for
this IRI
direction
none
Otherwise add:
predicate
full IRI
direction
forward
If present,
@rev
must contain one or more
resources, obtained according to the section on
CURIE

and IRI Processing
, each of which is added to the
local

list of incomplete triples
as follows:
predicate
full IRI
direction
reverse
The next step of the iteration is
to establish any
current property value
Predicates for the
current property
value
can be set by using
@property
. If
present, one or more resources are obtained according to the section on
CURIE and IRI 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 IRI
Processing
, and is not set to
XMLLiteral
in the vocabulary
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 IRI, as described in [
RDF-SYNTAX-GRAMMAR
], which will
have been obtained according to the section on
CURIE

and IRI Processing
otherwise
, as a
plain literal
if
@datatype
is present but has an empty value according to the section on
CURIE and IRI Processing
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.
otherwise
, as an
XML
literal
if
@datatype
is present and is
set to
XMLLiteral
in the vocabulary
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
of
XMLLiteral
in the vocabulary
The format of the resulting serialized content is as defined
in Exclusive XML Canonicalization Version 1.0 [
XML-EXC-C14N
].
Note
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
using
the value of
@content
if
@content
is
present.
otherwise
, if the
@rel
@rev
and
@content
attributes are
not
present,
as a resource
obtained from one of the following:
by using the resource from
@resource
, if present,
obtained according to the section on
CURIE

and IRI Processing
otherwise
, by using the
IRI
from
@href
if present, obtained according to the section on
CURIE

and IRI Processing
otherwise
, by using the
IRI
from
@src
if present, obtained according to the section on
CURIE

and IRI Processing
otherwise
, if
@typeof
is present and
@about
is not, the value of
typed resource
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-SYNTAX-GRAMMAR
]. 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 property value
is then used with each
predicate as follows:
If the element also includes the
@inlist
attribute, the
current property value
is added
to the
local list mapping
as follows:
if the
local list mapping
does not contain
a list associated with the predicate IRI, instantiate a
new list and add to local list mappings
add the
current property value
to the list
associated with the predicate IRI in the
local list
mapping
Otherwise the
current property value
is used
to generate a triple as follows:
subject
new subject
predicate
full IRI
object
current property value
If the
skip element
flag
is 'false',
and
new subject
was set to a
non-null value, then any
incomplete triple
within

the current context
should be completed:
The
list of incomplete triples
from the current
evaluation context
not
the
local list of incomplete triples
) will contain
zero or more predicate IRIs. This list is iterated over and each
of the predicates is used with
parent subject
and
new

subject
to generate a triple or add a new element to the
local list mapping
. Note that at each level there are
two
lists of
incomplete triple
s; 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 is used to determine what
will become the subject, and what will become the object, of each
generated triple:
If
direction
is 'none', the
new subject
is added to the list from the iterated
incomplete
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 'reverse'
then this is the triple generated:
subject
new subject
predicate
the predicate from the iterated
incomplete triple
object
parent subject
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 IRI 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 the
parent subject
of the current
evaluation context
the
list of IRI mappings
is set to the
local

list of IRI mappings
the
list of incomplete triples
is set to the
local list of incomplete triples
the
list mapping
is set to the
local
list mapping
language
is set to the value of
current

language
the
default vocabulary
is set to the value
of the
local default vocabulary
Finally, if there is one or more mapping in
the
local list mapping
, list triples are generated as
follows:
For each IRI in the
local list mapping
if the equivalent list does not exist in the
evaluation context
, indicating that the list was
originally instantiated on the current element, use the list as
follows:
If there are zero items in the list associated with the IRI,
generate the following triple:
subject
current subject
predicate
full IRI of the
local list mapping
associated with this list
object
Otherwise,
Create a new ‘bnode’ array containing newly created
bnode
s,

one for each item in the list
For each
bnode
-(
IRI
or literal) pair from the list
the following triple is generated:
subject
bnode
predicate
object
full IRI or literal
For each item in the ‘bnode’ array the following triple
is generated:
subject
bnode
predicate
object
next item in the ‘bnode’ array or, if that does not
exist,
A single additional triple is generated:
subject
current subject
predicate
full IRI of the
local list mapping
associated with this list
object
first item of the ‘bnode’ array
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.
The
processor graph
is designed as the 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.
rdfa:Warning
MUST
be generated when a CURIE prefix fails to be
resolved.
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 as
event-based

RDFa Processor
s. 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 the
processor
graph
. The callback
MAY
be the same as the one that is used
for the
output graph
as long as it can be determined
if a generated triple belongs in the
processor graph
or the
output graph
whole-graph RDFa Processor
is defined as any RDFa
Processor that processes the entire document and only
provides the
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
output

graph
, the
processor graph
, or both graphs as
a single, combined graph from the RDFa Processor.
If the graph preference is not specified, the
output graph
MUST
be returned.
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 IRI. For this
class of RDFa Processor, the software
MUST
allow the caller to
specify if they would like to retrieve the
output 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 are
output
processor
or both
values, in any order, separated by a comma character.
If the graph preference is not specified, the
output 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 IRI 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):
Example 45
@prefix rdfa: .
@prefix xsd: .
@prefix dc: .
[] a rdfa:DocumentError ;
dc:description "The document could not be parsed due to parsing errors." ;
dc: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 (note that a
processor may not have these information in all cases, i.e., these
rdfa:context
information are not required):
Example 46
@prefix rdfa: .
@prefix xsd: .
@prefix dc: .
@prefix ptr: .
@prefix ht: .

[] a rdfa:DocumentError ;
dc:description "The document could not be parsed due to parsing errors." ;
dc:date "2010-06-30T13:40:23"^^xsd:dateTime ;
rdfa:context ;
rdfa:context [
a ptr:Pointer ;
# Detailed xpointer/xpath information provided here to locate the error.
] ;
rdfa:context [
a ht:Response ;
ht:responseCode
# The HTTP response headers on the request for the source file.
].
7.7
Vocabulary Expansion
Processors
MAY
perform vocabulary expansion by
utilizing limited RDFS and OWL entailment rules,
as described in
RDFa

Vocabulary Expansion
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.
Note
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 the
evaluation

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 IRI 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:
Example 47







...


This would generate the following triples:
Example 48
<> foaf:primaryTopic <#bbq> .
<> dc:creator "Jo" .
It is possible for the data to appear elsewhere in the document:
Example 49

Jo
's blog

Welcome to my blog.

which would still generate the triple:
Example 50
<> dc:creator "Jo" .
In (X)HTML the value of
base
may change the initial
value of
current subject
Example 51








...


An RDFa Processor should now generate the following triples,
regardless of the IRI from which the document is served:
Example 52
foaf:primaryTopic .
dc:creator "Jo" .
8.1.1.2
Using
@about
As processing progresses, any
@about
attributes will
change the
current subject
. The value of
@about
is an IRI or a CURIE. If it is a relative IRI 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:
Example 53
prefix="cal: http://www.w3.org/2002/12/cal/ical#">






about="#bbq"
typeof="cal:Vevent"
I'm holding
property="cal:summary"
one last summer barbecue
,
on
property="cal:dtstart"
content="2015-09-16T16:00:00-05:00"
datatype="xsd:dateTime">
September 16th at 4pm
.

With this markup an RDFa Processor will generate the following
triples:
Example 54
<> foaf:primaryTopic <#bbq> .
<> dc: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:
Example 55
John knows
about="mailto:john@example.org"
rel="foaf:knows" href="mailto:sue@example.org">Sue.

Sue knows
about="mailto:sue@example.org"
rel="foaf:knows" href="mailto:jim@example.org">Jim.
This should generate the following triples:
Example 56
foaf:knows .
foaf:knows .
Similarly, authors may make statements about images:
Example 57
about="photo1.jpg"
this photo was taken by
Mark Birbeck

which should generate the following triple:
Example 58
dc:creator "Mark Birbeck" .
8.1.1.3
Typing resources with
@typeof
@typeof
defines typing triples.
@typeof
works differently to other ways of setting a predicate since the
predicate is always
rdf:type
, which means that the
processor only requires the value of the type. The
question is: which resource gets these typing information?
If the element has an
@about
, which creates a new
context for statements, the typing relationships are defined on
that resource. For example, the following:
Example 59
about="http://dbpedia.org/resource/Albert_Einstein" typeof="foaf:Person"
Albert Einstein
Albert

also creates the triple:
Example 60
rdf:type foaf:Person .
The
@about
attribute is the main source for typing;
if it is present on an element, it determines the effect of
@typeof
with the highest priority. If
@about
is
not
present, but the element is used only to define possible subject
resources via, e.g.,
@resource
(i.e., there is
no
@rel
@rev
, or
@property
present), then that resource is used for the typed resource, just
like
@about
If an
@rel
is present (and still no
@about
then the explicit object of the triples defined by
@rel
is typed. For example, in the case of:
Example 61

resource="http://dbpedia.org/resource/German_Empire"
typeof="http://schema.org/Country"

the generated triples also include:
Example 62
rdf:type .
Finally,
@typeof
also has the additional feature of
creating a new context for statements,
in case no other
attributes define any
. This involves generating a new
bnode
(see below for more about bnodes). 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:
Example 63
typeof="foaf:Person"
Albert Einstein
Albert

This markup would cause a
bnode
to be created
which has a 'type' of
foaf:Person
, as well as name
and given name properties:
Example 64
_:a rdf:type foaf:Person .
_:a foaf:name "Albert Einstein" .
_:a foaf:givenName "Albert" .
This usage of “isolated”
@typeof
may be viewed as a shorthand for:
Example 65
resource="_:a" typeof="foaf:Person"
Albert Einstein
Albert

Similarly,
Example 66

typeof="http://schema.org/Country"
the German Empire

generates:
Example 67
dbp:birthPlace _:b .
_:b dbp:conventionalLongName "the German Empire" .
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.
8.1.1.3.1
Chaining with
@property
and
@typeof
As emphasized in the
section on chaining
one of the main differences between
@property
and
@rel
(or
@rev
) is that the former does not induce
chaining. The
only
exception to this rule is when
@typeof
is also present on the element. In that case the effect of
@property
is identical to
@rel
. For example, the previous
example could have been written as:
Example 68

typeof="http://schema.org/Country"
the German Empire

generating the same triples as before. Here again, a
@typeof
without
an
@about
or a
@resource
can be regarded as a shorthand
for an additional
@resource
attribute referring to the identifier of a fresh
bnode
8.1.1.4
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.
8.1.1.4.1
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:
Example 69

Albert Einstein
1879-03-14

property="dbp:conventionalLongName">the German Empire

In an earlier illustration the subject and object for the
German Empire were connected by removing the
@resource
relying on the
@about
to set the object:
Example 70

Albert Einstein
1879-03-14

property="dbp:conventionalLongName">the German Empire

but it is also possible for authors to achieve the same effect
by removing the
@about
and leaving the
@resource
Example 71

Albert Einstein
1879-03-14

resource="http://dbpedia.org/resource/German_Empire"
the German Empire

In this situation, all statements that are 'contained' by the
object resource representing the German Empire (the value in
@resource
will have the same subject, making it easy for authors to add
additional statements:
Example 72

Albert Einstein
1879-03-14

the German Empire

Looking at the triples that an RDFa Processor would generate,
we can see that we actually have two groups of statements; the
first group is set to refer to the
@about
that
contains them:
Example 73
foaf:name "Albert Einstein" .
dbp:dateOfBirth "1879-03-14"^^xsd:date .
dbp:birthPlace .
while the second group refers to the
@resource
that contains them:
Example 74

dbp:conventionalLongName "the German Empire" .

dbp-owl:capital .
Note also that the same principle described here applies to
@src
and
@href
8.1.1.4.2
Inheriting an anonymous subject
There will be occasions when the author wants to connect the
subject and object as shown above, but is not concerned to name
the resource that is common to the two statements (i.e., the
object of the first statement, which is the subject of the
second). For example, to indicate that Einstein was influenced
by Spinoza the following markup could well be used:
Example 75

Albert Einstein
1879-03-14

An RDFa Processor will generate the following triples:
Example 76

dbp-owl:influenced .
foaf:name "Albert Einstein" .
dbp:dateOfBirth "1879-03-14"^^xsd:date .
However, an author could just as easily say that Spinoza
influenced
something by the name of Albert Einstein, that
was born on March 14th, 1879
Example 77

Albert Einstein
1879-03-14

In RDF terms, the item that 'represents' Einstein is
anonymous
since it has no IRI to identify it. However, the item is given
an automatically generated
bnode
, and it is onto
this identifier that all child statements are attached:
An RDFa Processor will generate the following triples:
Example 78
dbp-owl:influenced _:a .
_:a foaf:name "Albert Einstein" .
_:a dbp:dateOfBirth "1879-03-14"^^xsd:date .
Note that the
div
is superfluous, and an RDFa
Processor will create the intermediate object even if the
element is removed:
Example 79

Albert Einstein
1879-03-14

An alternative pattern is to
keep
the
div
and move the
@rel
onto it:
Example 80

rel="dbp-owl:influenced"
Albert Einstein
1879-03-14

From the point of view of the markup, this latter layout is to
be preferred, since it draws attention to the 'hanging rel'. But
from the point of view of an RDFa Processor, all of these
permutations need to be supported.
8.2
Completing incomplete triples
When a new subject is calculated, it is also used to complete any
incomplete triples that are pending. This situation arises when the
author wants to 'chain' a number of statements together. For example,
an author could have a statement that Albert Einstein was born in the
German Empire:
Example 81

Albert Einstein
1879-03-14

and then a further statement that the 'long name' for this country
is
the German Empire
Example 82
property="dbp:conventionalLongName">the German Empire
RDFa allows authors to insert this statement as a self-contained
unit into other contexts:
Example 83

Albert Einstein
1879-03-14

property="dbp:conventionalLongName">the German Empire

But it also allows authors to avoid unnecessary repetition and to
'normalize' out duplicate identifiers, in this case the one for the
German Empire:
Example 84

Albert Einstein
1879-03-14

property="dbp:conventionalLongName">the German Empire

When this happens the
@rel
for 'birth place' is
regarded as a 'hanging rel' because it has not yet generated any
triples, but these 'incomplete triples' are completed by the
@about
that appears on the next line. The first step is therefore to store
the two parts of the triple that the RDFa Processor
does
have, but without an object:
Example 85
dbp:birthPlace
Then as processing continues, the RDFa Processor encounters the
subject of the statement about the long name for the German Empire,
and this is used in two ways. First it is used to complete the
'incomplete triple':
Example 86

dbp:birthPlace

and second it is used to generate its own triple:
Example 87

dbp:conventionalLongName "the German Empire" .
Note that each occurrence of
@about
will complete any
incomplete triples. For example, to mark up the fact that Albert
Einstein had a residence both in the German Empire and Switzerland, an
author need only specify one
@rel
value that is then used
with multiple
@about
values:
Example 88

In this example there is one incomplete triple:
Example 89
dbp-owl:residence
When the processor meets each of the
@about
values,
this triple is completed, giving:
Example 90

dbp-owl:residence


dbp-owl:residence

These examples show how
@about
completes triples, but
there are other situations that can have the same effect. For example,
when
@typeof
creates a new
bnode
(as
described above), that will be used to complete any 'incomplete
triples'. To indicate that Spinoza influenced both
Einstein and Schopenhauer, the following markup could be used:
Example 91

typeof="foaf:Person"
Albert Einstein
1879-03-14

typeof="foaf:Person"
Arthur Schopenhauer
1788-02-22

First the following incomplete triple is stored:
Example 92
dbp-owl:influenced
Then when the RDFa Processor processes the two occurrences of
@typeof
each generates a
bnode
, which is used to both complete
the 'incomplete triple', and to set the subject for further
statements:
Example 93
dbp-owl:influenced
_:a
_:a
rdf:type foaf:Person .
_:a
foaf:name "Albert Einstein" .
_:a
dbp:dateOfBirth "1879-03-14"^^xsd:date .
dbp-owl:influenced
_:b
_:b
rdf:type foaf:Person .
_:b
foaf:name "Arthur Schopenhauer" .
_:b
dbp:dateOfBirth "1788-02-22"^^xsd:date .
Triples are also 'completed' if any one of
@property
@rel
or
@rev
are present. However, unlike the situation when
@about
or
@typeof
are present, all predicates are attached to
one
bnode
Example 94

Albert Einstein
1879-03-14

This example has two 'hanging rels', and so two situations when
'incomplete triples' will be created. Processing would proceed as
follows; first an incomplete triple is stored:
Example 95
dbp-owl:influenced
Next, the RDFa Processor processes the predicate values for
foaf:name
dbp:dateOfBirth
and
dbp-owl:residence
, but
note that only the first needs to 'complete' the 'hanging rel'. So
processing
foaf:name
generates two triples:
Example 96
dbp-owl:influenced
_:a
_:a
foaf:name "Albert Einstein" .
but processing
dbp:dateOfBirth
generates only one:
Example 97
_:a
dbp:dateOfBirth "1879-03-14"^^xsd:date .
Processing
dbp-owl:residence
also uses the same
bnode
but note that it also generates its own 'incomplete triple':
Example 98
_:a dbp-owl:residence
As before, the two occurrences of
@about
complete the
'incomplete triple', once each:
Example 99
_:a dbp-owl:residence

_:a dbp-owl:residence

The entire set of triples that an RDFa Processor should
generate is as follows:
Example 100
dbp-owl:influenced
_:a
_:a
foaf:name "Albert Einstein" .
_:a
dbp:dateOfBirth "1879-03-14"^^xsd:date .
_:a
dbp-owl:residence .
_:a
dbp-owl:residence .
8.3
Object resolution
Although objects have been discussed in the previous sections, as
part of the explanation of subject resolution, chaining, evaluation
contexts, and so on, this section will look at objects in more detail.
There are two types of object,
IRI resource
s and
literal
s.
literal
object can be set by
@content
or the inline
text of element if
@property
to express a
predicate
Note that the use of
@content
prohibits the inclusion of
rich markup in your literal. If the inline content of an element
accurately represents the object, then documents should rely upon
that rather than duplicating that data using the
@content
An
IRI resource
object can be set using one of
@rel
or
@rev
to express a
predicate
, and then
either
using one of
@href
@resource
or
@src
to provide an object resource explicitly,
or
using the
chaining techniques described above to obtain an object from a nested
subject, or from a
bnode
Alternatively
, the
@property
can also be used to define an IRI resource; this requires the presence of a
@resource
@href
, or
@src
and
the
absence of
@rel
@rev
@datatype
or
@content
8.3.1
Object resolution for the
@property
attribute
An
object literal
will be generated when
@property
is present and no resource attribute is present.
@property
provides the predicate, and the
following sections describe how the actual literal to be generated
is determined.
8.3.1.1
Plain Literals
@content
can be used to indicate a
plain
literal
, as follows:
Example 101
property="dc:creator"
content="Mark Birbeck"
/>
The
plain literal
can also be specified by using
the content of the element:
Example 102
property="dc:creator">
Mark Birbeck

Both of these examples give the following triple:
Example 103
dc:creator "Mark Birbeck" .
The value of
@content
is given precedence over any
element content, so the following would give exactly the same
triple as shown above:
Example 104
property="dc:creator"
content="Mark Birbeck"
>John Doe
8.3.1.1.1
Language Tags
RDF allows
plain literal
s to have a language tag,
as illustrated by the following example from [
RDF11-TESTCASES
]:
Example 105

"chat"
@fr
In RDFa the Host Language may provide a mechanism for setting
the language tag. In XHTML+RDFa [
XHTML-RDFA
], for example,
the XML language attribute
@xml:lang
or the attribute
@lang
is used to add
this information, whether the plain literal is designated by
@content
or by the inline text of the element:
Example 106
property="ex:property"
xml:lang="fr"
content="chat" />
Note that the language value can be inherited as defined in
XML10-4e
], so the following syntax will give the same triple
as above:
Example 107
prefix="ex: http://www.example.com/ns/"
xml:lang="fr"


property="ex:property" content="chat" />

...

8.3.1.2
Typed Literals
Literals can be given a data type using
@datatype
This can be represented in RDFa as follows:
Example 108
datatype="xsd:dateTime"
September 16th at 4pm
.
The triple that this markup generates includes the datatype after
the literal:
Example 109
<> cal:dtstart "2015-09-16T16:00:00-05:00"^^
xsd:dateTime
8.3.1.3
XML Literals
XML documents cannot contain XML markup in their attributes,
which means it is not possible to represent XML within
@content
(the following would cause an XML parser to generate an error):
Example 110

content="E = mc²: The Most Urgent Problem of Our Time"
/>

RDFa therefore supports the use of arbitrary markup to express XML
literals by using
@datatype
Example 111

E = mc²: The Most Urgent Problem of Our Time

This would generate the following triple, with the XML preserved
in the literal:
Example 112
<> dc:title "E = mc²: The Most Urgent Problem of Our Time"^^rdf:XMLLiteral .
Note
This requires that an IRI mapping for the prefix
rdf
has been defined.
In the examples given here the
sup
element is
actually part of the meaning of the literal, but there will be
situations where the extra markup means nothing, and can therefore
be ignored. In this situation omitting the
@datatype
attribute or specifying an empty
@datatype
value can
be used to create a plain literal:
Example 113

You searched for Einstein:

datatype=""
>Albert Einstein
(b. March 14, 1879, d. April 18, 1955) was a German-born theoretical physicist.

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 triple:
Example 114
foaf:name
"Albert Einstein"
Note
The value of this
XML
Literal
is the exclusive canonicalization
XML-EXC-C14N
] of the RDFa element's value.
8.3.2
IRI object resolution
Most of the rules governing the processing of objects that are
resources are to be found in the processing descriptions given
above, since they are important for establishing the subject. This
section aims to highlight general concepts, and anything that might
have been missed.
One or more
IRI object
s are needed when
@rel
or
@rev
is present. Each
attribute will cause triples to be generated when used with
@href
@resource
or
@src
, or with the subject
value of any nested statement if none of these attributes are
present.
If
@rel
or
@rev
is not present, and neither is
@datatype
or
@content
, a
@property
attribute
will cause triples to be generated when used with
@href
@resource
or
@src
(See also the
section on
@property
and
@typeof
for an additional special case involving
@property
.)
@rel
and
@rev
are essentially the
inverse of each other; whilst
@rel
establishes a
relationship between the
current subject
as subject,
and the
current object resource
as the object,
@rev
does the exact opposite, and uses the
current object resource
as the subject, and the
current subject
as the object.
8.3.2.1
Using
@resource
to set the object
RDFa 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:
Example 115

resource="urn:ISBN:0140449132"

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!

The
blockquote
element generates the following
triple:
Example 116

.
Note that, in the example above,
@property
could
have been used instead of
@rel
, yielding the same
triple.
8.3.2.2
Using
@href
or
@src
to set the object
If no
@resource
is present, then
@href
or
@src
are next in priority order for setting the
object.
When a predicate has been expressed using
@rel
, the
@href
or
@src
on the RDFa statement's
element is used to identify the object with a
IRI reference
Their types are an IRI:
Example 117
rel="foaf:knows" href="mailto:sue@example.org"
/>
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 other,
for example when a picture is taken
by
Mark, but that
picture also
depicts
him:
Example 118
src="photo1.jpg"
rev="dc:creator" rel="foaf:img"
/>
which then yields two triples:
Example 119

dc:creator .

foaf:img .
8.3.2.3
Incomplete triples
When a triple predicate has been expressed using
@rel
or
@rev
, but no
@href
@src
or
@resource
exists on the same element, there is a
'hanging rel'. This causes the current subject and all possible
predicates (with an indicator of whether they are 'forwards, i.e.,
@rel
values, or not, i.e.,
@rev
values),
to be stored as 'incomplete triples' pending discovery of a
subject that could be used to 'complete' those triples.
This process is described in more detail in
Completing

'Incomplete Triples'
8.4
List Generation
An RDF graph is a collection of triples. This also means that if the
graph contains two triples sharing the same subject and predicate:
Example 120
"first object", "second object" ;
There is no way for an application to rely on the relative order of
the two triples when, for example, querying a database containing
these triples. For most of the applications and data sets this is not
a problem, but, in some cases, the order is important. A typical case
is publications: when a book or an article has several co-authors, the
order of the authors may be important.
RDF has a set of predefined predicates that have an agreed-upon
semantic of order. For example, the publication: "Semantic Annotation
and Retrieval, by Ben Adida, Mark Birbeck, and Ivan Herman" could be
described in RDF triples using these terms as follows:
Example 121
@prefix bibo: .
@prefix dc: .
@prefix rdf: .
[ a bibo:Chapter ;
dc:title "Semantic Annotation and Retrieval" ;
dc:creator [
rdf:first rdf:rest [
rdf:first ;
rdf:rest [
rdf:first ;
rdf:rest rdf:nil .
] .
] .
] .
...
which conveys the notion of 'order' for the three authors.
Admittedly, this is not very readable. However, Turtle has a
syntactic shorthand for these structures:
Example 122
@prefix bibo: .
@prefix dc: .
@prefix rdf: .
[ a bibo:Chapter ;
dc:title "Semantic Annotation and Retrieval" ;
dc:creator
(


) .
...
It would of course be possible to reproduce the same structure in
RDFa, using the RDF predicates
rdf:first
rdf:rest
as well as the special resource
rdf:nil
. However, to
make this easier, RDFa provides the
@inlist
. What this
attribute signals is that the object generated on that element should
be put on a list; the list is used with the common predicate
and subject. Here is how the previous structure could look in
RDFa:
Example 123

"Semantic Annotation and Retrieval" by
inlist=""
property="dc:creator"
href="http://ben.adida.net/#me"
>Ben Adida,
inlist=""
property="dc:creator"
href="http://twitter.com/markbirbeck"
>Mark Birbeck, and
inlist=""
property="dc:creator"
href="http://www.ivan-herman.net/foaf#me"
>Ivan Herman.

Note that the order in the list is determined by the document order.
(The value of the
@inlist
is not relevant, only its
presence is.)
Lists may also include IRIs and not only literals. For example, two
of the three co-authors could decide to publicise their FOAF address
in the authors’ list:
Example 124

"Semantic Annotation and Retrieval", by
inlist=""
property="dc:creator"
resource="http://ben.adida.net/#me"
>Ben Adida,
inlist=""
property="dc:creator"
>Mark Birbeck, and
inlist=""
property="dc:creator"
resource="http://www.ivan-herman.net/foaf#me"
>Ivan Herman.

yielding:
Example 125
@prefix bibo: .
@prefix dc: .
@prefix rdf: .
[ a bibo:Chapter ;
dc:title "Semantic Annotation and Retrieval" ;
dc:creator ( "Mark Birbeck" ) .
...
In the example above,
@rel
could have been used leading
exactly to the same triples:
Example 126

"Semantic Annotation and Retrieval", by
inlist=""
rel="dc:creator"
resource="http://ben.adida.net/#me"
>Ben Adida,
inlist=""
property="dc:creator"
>Mark Birbeck, and
inlist=""
rel="dc:creator"
resource="http://www.ivan-herman.net/foaf#me"
>Ivan Herman.

Incomplete Triples
can also be
used in conjunction with lists when all list elements are resources
and not literals. For example, the previous example, this time with all
three authors referring to their FOAF profile, could have been written
as:
Example 127

"Semantic Annotation and Retrieval", by
rel="dc:creator"
inlist=""
href="http://ben.adida.net/#me"
>Ben Adida,
href="http://internet-apps.blogspot.com/2008/03/my-profile.html#me"
>Mark Birbeck, and
href="http://www.ivan-herman.net/foaf#me"
>Ivan Herman.

Resulting in:
Example 128
@prefix bibo: .
@prefix dc: .
@prefix rdf: .
[ a bibo:Chapter ;
dc:title "Semantic Annotation and Retrieval" ;
dc:creator (

) .
...
Note that it is also possible to express an empty list,
without
@inlist
, using:
Example 129

9.
RDFa Initial Contexts
RDFa permits Host Languages to define an
initial context
Such a context is a collection of terms, prefix mappings, and/or a default
vocabulary declaration. An initial context is either intrinsically
known to the parser, or it is loaded as external documents and
processed. These documents
MUST
be defined in an approved RDFa Host
Language (currently XML+RDFa, XHTML+RDFa [
XHTML-RDFA
], and HTML+RDFa [
HTML-RDFA
]).
They
MAY
also be defined in other formats (e.g., RDF/XML
RDF-SYNTAX-GRAMMAR
], or Turtle [
TURTLE
]).
When an initial
context document is processed, it is evaluated as follows:
Parse the content (according to the processing rules for that
document type) and extract the triples into a collection associated
with that IRI. Note: These triples
MUST NOT
be co-mingled with the
triples being extracted from any other IRI.
For every subject with a pair of predicates that have the values
rdfa:prefix
and
rdfa:uri
, create a key-value mapping from the
rdfa:prefix
object literal (the key) to the
rdfa:uri
object literal
(the value). Add this mapping to the
list of IRI mappings
of the
initial evaluation context
, after
transforming the 'prefix' component to lower-case.
For every subject with a pair of predicates that have the values
rdfa:term
and
rdfa:uri
, create a key-value mapping from the
rdfa:term
object literal (the key) to the
rdfa:uri
object literal
(the value). Add this mapping to the
term mappings
of
the
initial evaluation context
For an extracted triple that has a predicate of
rdfa:vocabulary
define the
default vocabulary
of the
initial

evaluation context
to be the object literal of the
rdfa:vocabulary
predicate.
When an RDFa Initial Context is defined using an RDF serialization, it
MUST
use the vocabulary terms above to declare the components of the
context.
Note
Caching of the relevant triples retrieved via this
mechanism is
RECOMMENDED
. Embedding definitions for well known, stable
RDFa Initial Contexts in the implementation is
RECOMMENDED
Note
The object literal for the
rdfa:uri
predicate
MUST
be an absolute IRI.
The object literal for the
rdfa:term
predicate
MUST
match the production for
term
The
object literal for the
rdfa:prefix
predicate must match
the production for
prefix
The object literal
for the
rdfa:vocabulary
predicate
MUST
be an
absolute IRI.
If one of the objects is not a literal, does not match its associated
production, if there is more than one
rdfa:vocabulary
predicate, or if there are additional
rdfa:uri
or
rdfa:term
predicates sharing the same subject, an RDFa Processor
MUST NOT
create
the associated mapping.
10.
RDFa Vocabulary Expansion
Since RDFa is based on RDF, the semantics of RDF vocabularies can be
used to gain more knowledge about data. Vocabularies, properties and
classes are identified by IRIs, which enables them to be discoverable.
RDF data published at the location of these IRIs can be retrieved, and
descriptions of the properties and classes using specified semantics can
be applied.
RDFa Vocabulary Expansion
is an optional processing step which may be
added once the normal processing steps described in
Processing

Model
are complete. Vocabulary expansion relies on a very small
sub-set of OWL entailment [
OWL2-OVERVIEW
] to add triples to the
output

graph
based on rules and property/class relationships described
in referenced vocabularies. Vocabulary expansion
MAY
be performed as
part of a larger RDF toolset including, for example, an OWL 2 RL
reasoner. Alternatively, using vocabulary data added to the
output
graph
in processing step 2 of
Sequence
expansion
MAY
also be done using a separate and dedicated (e.g., rule
based) reasoner after the
output graph
has been generated,
or as the last processing step by an RDFa processor.
It can be very useful to make generalized data available for
subsequent usage of RDFa-embedded data by expanding inferred statements
entailed by these semantics. This provides for existing vocabularies
that extend well-known vocabularies to have those properties added to
the output graph automatically. For example, the namespace document of
the Creative Commons vocabulary, i.e.,
defines
cc:license
to be a sub-property of
dc:license
By using the
@vocab
attribute, one can describe a licensing
information as follows:
Example 130
This document is licensed under the
rel="license"
href="http://creativecommons.org/licenses/by-nc-nd/3.0/">
Creative Commons By-NC-ND License
.
which results in the following
output graph
Example 131
@prefix cc: .
@prefix rdfa: .
<> cc:license ;
rdfa:usesVocabulary .
After vocabulary expansion, the
output graph
contains:
Example 132
@prefix cc: .
@prefix rdfa: .
@prefix dc: .
<> cc:license ;
dc:license
rdfa:usesVocabulary .
Other vocabularies, specifically intended to provide relations to
multiple vocabularies, could also be defined by publishers, allowing use
of terms in a single namespace which result in properties and/or classes
from other primary vocabularies being imported. This benefits publishers
as data is now more widely searchable and encourages the practice of
referencing well-known vocabularies.
10.1
Details of the RDFa Vocabulary Expansion
This section is non-normative.
Once the
output graph
is generated following the
processing steps defined in
Sequence
processors
MAY
perform the following processing steps on the output
graph. It must do so only if the user of the processor explicitly asks
for it, as prescribed in
Vocabulary
Expansion Control of RDFa Processors
vocabulary graph
is created as follows:
Each object IRI in the
output graph
that has a subject the current
document (
base
) IRI and a predicate of
rdfa:usesVocabulary
is dereferenced.
If the dereferencing yields the serialization of an RDF
graph, that serialization is parsed and the resulting graph is merged
with the vocabulary graph. (An RDFa processor capable of vocabulary
expansion
MUST
accept an RDF graph serialized in RDFa, and
SHOULD
accept other standard serialization formats of RDF such as RDF/XML
RDF-SYNTAX-GRAMMAR
] and Turtle [
TURTLE
].)
Note
Note that if, in the second step, a particular
vocabulary is serialized in RDFa, that particular graph is not
expected to undergo any vocabulary expansion on its own.
Vocabulary expansion is then performed as follows:
The processor operates on the merge of the default and vocabulary
graphs using
RDFa Vocabulary
Entailment
Add the new triples inferred from the
output graph
using this entailment to the (expanded)
output graph
The processor
SHOULD NOT
add the triples appearing in the
vocabulary
graph
only.
The goal of the second step is to avoid adding
the "axioms", e.g., the sub-property definitions to the output graph.
Applications usually do not require any of this additional information.
10.1.1
RDFa Vocabulary Entailment
For the purpose of vocabulary processing, RDFa used a very
restricted subset of the OWL vocabulary and is based on the RDF-Based
Semantics of OWL [
OWL2-RDF-BASED-SEMANTICS
]. The RDFa
Vocabulary Entailment uses the following terms:
rdf:type
rdfs:subClassOf
rdfs:subPropertyOf
owl:equivalentClass
owl:equivalentProperty
Note
RDFa Vocabulary Entailment considers only the entailment on individuals
(i.e., not on the relationships that can be deduced on the
properties or the classes themselves.)
Note
While the formal definition of the RDFa Entailment
refers to the general OWL 2 Semantics, practical implementations may
rely on a subset of the OWL 2 RL Profile’s entailment expressed in
rules (
section

4.3
of [
OWL2-PROFILES
]). The
relevant rules are, using the rule identifications in
section

4.3
of [
OWL2-PROFILES
]):
prp-spo1
prp-eqp1
prp-eqp2
cax-sco
cax-eqc1
and
cax-eqc2
The entailment described in this section is the
minimum
useful level for RDFa. Processors may, of course, choose to follow
more powerful entailment regimes, e.g., include full RDFS [
RDF11-MT
or OWL [
OWL2-OVERVIEW
] entailments. Using those entailments
applications may perform datatype validation by checking
rdfs:range
of a property, or use the advanced facilities offered by, e.g., OWL’s
property chains to interlink vocabularies further.
10.2
Vocabulary Expansion Control of RDFa Processors
Conforming RDFa processors are not required to provide vocabulary
expansion.
If an RDFa processor provides vocabulary expansion, it
MUST NOT
be
performed by default. Instead, the processor
MUST
provide an option,
vocab_expansion
which, when used, instructs the RDFa processor to perform a vocabulary
expansion before returning the output graph.
Note
Although vocabulary expansion is described in terms of
vocabulary graph
and OWL 2 entailment rules, processors
are free to use any process which obtains equivalent results.
10.2.1
Notes to RDFa Vocabulary Implementations and Publishing
This section is non-normative.
For RDFa Processors caching the relevant graphs retrieved via this
mechanism is
RECOMMENDED
. Caching is usually based on HTTP response
headers like expiration time, cache control, etc.
For publishers of vocabularies, the IRI for the vocabularies
SHOULD
be dereferenceable, and should return an RDF graph with the
vocabulary description. This vocabulary description
SHOULD
be
available encoded in RDFa, and
MAY
also be available in other RDF
serialization syntaxes (using content negotiation to choose among
the different formats). If possible, vocabulary descriptions
SHOULD
include subproperty and subclass statements linking the vocabulary
terms to other, well-known vocabularies. Finally, HTTP responses
SHOULD
include fields usable for cache control, e.g., expiration
date.
A.
CURIE Datatypes
In order to facilitate the use of CURIEs in markup languages, this
specification defines some additional datatypes in the XHTML datatype
space (
). Markup
languages that want to import these definitions can find them in the
"datatypes" file for their schema grammar:
DTD
xhtml-datatypes.mod
XML
Schema xhtml-datatypes.xsd
Specifically, the following datatypes are defined:
CURIE
A single
curie
CURIEs
A white space separated list of
CURIE
CURIEorIRI
CURIE
or an
IRI
CURIEorIRIs
A white space separated list of
CURIEorIRI
SafeCURIE
A single
safe_curie
SafeCURIEorCURIEorIRI
A single
SafeCURIE
or
CURIEorIRI
SafeCURIEorCURIEorIRIs
A white space separated list of
SafeCURIEorCURIEorIRI
s.
TERM
A single
term
TERMorCURIEorAbsIRI
TERM
or a
CURIEorIRI
TERMorCURIEorAbsIRIs
A white space separated list of
TERMorCURIEorAbsIRI
A.1
XML Schema Definition
This section is non-normative.
The following
informative
XML Schema definition for these
datatypes is included as an example:
Example 133

xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns="http://www.w3.org/1999/xhtml/datatypes/"
xmlns:xh11d="http://www.w3.org/1999/xhtml/datatypes/"
targetNamespace="http://www.w3.org/1999/xhtml/datatypes/"
elementFormDefault="qualified"

A.2
XML DTD Definition
This section is non-normative.
The following
informative
XML DTD definition for these
datatypes is included as an example:
Example 134








B.
The RDFa Vocabulary
The RDFa Vocabulary has three roles: it contains the predicates to
define the terms and prefixes in
initial context
documents, it contains the classes and predicates for the messages that
processor graph
may contain and, finally, it contains
the predicate necessary for vocabulary processing. The IRI of the
vocabulary is
; the usual prefix
used in this document is
rdfa
This vocabulary specification is available in
XHTML+RDFa

1.1
Turtle
, and in
RDF/XML
formats.
B.1
Term and Prefix Assignments
The RDFa Vocabulary includes the following triples (shown here in
Turtle [
TURTLE
] format):
Example 135
@prefix dc: .
@prefix owl: .
@prefix rdf: .
@prefix rdfs: .
@prefix rdfa: .
@prefix foaf: .

a owl:Ontology .

rdfa:PrefixOrTermMapping a rdfs:Class, owl:Class ;
dc:description "The top level class for prefix or term mappings." .

rdfa:PrefixMapping dc:description "The class for prefix mappings." .
rdfs:subClassOf rdfa:PrefixOrTermMapping .

rdfa:TermMapping dc:description "The class for term mappings." .
rdfs:subClassOf rdfa:PrefixOrTermMapping .

rdfa:prefix a rdf:Property, owl:DatatypeProperty ;
rdfs:domain rdfa:PrefixMapping ;
dc:description "Defines a prefix mapping for an IRI; the value is supposed to be a NMTOKEN." .

rdfa:term a rdf:Property, owl:DatatypeProperty ;
rdfs:domain rdfa:TermMapping ;
dc:description "Defines a term mapping for an IRI; the value is supposed to be a NMTOKEN." .

rdfa:uri a rdf:Property, owl:DatatypeProperty ;
rdfs:domain rdfa:PrefixOrTermMapping ;
dc:description """Defines the IRI for either a prefix or a term mapping;
the value is supposed to be an absolute IRI.""" .

rdfa:vocabulary a rdf:Property, owl:DatatypeProperty ;
dc:description """Defines an IRI to be used as a default vocabulary;
the value is can be any string; for documentation purposes it is advised to use
the string ‘true’ or ‘True’.""" .
These predicates can be used to define the
initial context
for a given Host Language.
These predicates are used to 'pair' IRI strings and their usage in
the form of a prefix and/or a term as part of, for example, a blank
node. An example can be as follows:
Example 136
[] rdfa:uri "http://xmlns.com/foaf/0.1/name" ;
rdfa:prefix "foaf" .
which defines a prefix for the FOAF IRI.
B.2
Processor Graph Reporting
The Vocabulary includes the following term definitions (shown here in
Turtle [
TURTLE
] format):
Example 137
@prefix dc: .
@prefix owl: .
@prefix rdf: .
@prefix rdfs: .
@prefix rdfa: .

rdfa:PGClass a rdfs:Class, owl:Class;
dc:description "The top level class of the hierarchy." .

rdfa:Error dcterms:description "The class for all error conditions.";
rdfs:subClassOf rdfa:PGClass .

rdfa:Warning dcterms:description "The class for all warnings.";
rdfs:subClassOf rdfa:PGClass .

rdfa:Info dcterms:description "The class for all informations.";
rdfs:subClassOf rdfa:PGClass .

rdfa:DocumentError dc:description "An error condition to be used when the document
fails to be fully processed as a result of non-conformant host language markup.";
rdfs:subClassOf rdfa:Error .

rdfa:VocabReferenceError dc:description "A warning to be used
when the value of a @vocab attribute cannot be dereferenced, hence the vocabulary expansion
cannot be completed.";
rdfs:subClassOf rdfa:Warning .

rdfa:UnresolvedTerm dc:description "A warning to be used when a Term fails to be resolved.";
rdfs:subClassOf rdfa:Warning .

rdfa:UnresolvedCURIE dc:description "A warning to be used when a CURIE prefix
fails to be resolved.";
rdfs:subClassOf rdfa:Warning .

rdfa:context a owl:ObjectProperty, rdf:Property;
dc:description "Provides extra context for the error, e.g., http response,
an XPointer/XPath information, or simply the IRI that created the error.";
rdfs:domain rdfa:PGClass .
B.3
Term for vocabulary expansion
The Vocabulary includes the following term definitions (shown here in
Turtle [
TURTLE
] format):
Example 138
@prefix dc: .
@prefix owl: .
@prefix rdf: .
@prefix rdfa: .

rdfa:usesVocabulary a owl:ObjectProperty, rdf:Property;
dc:description "Provides a relationship between the host document and a vocabulary
defined using the @vocab facility of RDFa1.1." .
C.
Changes
This section is non-normative.
C.1
Major differences since the Last Published Recommentation
References to the other RDFa 1.1 documents, as well as to RDF 1.1 documents, have been updated
A minor clarification has been added to section 4.1. to the processors can return processor and output graphs
C.2
Major differences with RDFa Syntax 1.0
This specification introduces a number of new features, and extends
the behavior of some features from the previous version. The
following summary may be helpful to RDFa Processor developers, but
is
not
meant to be comprehensive.
Specific rules about XHTML have been moved into a companion
specification: [
XHTML-RDFA
].
Prefix mappings can now be declared using
@prefix
in addition to
@xmlns
. The usage of
@xmlns
has been deprecated.
Prefix names are now required to be converted to lower-case when
the mapping is defined. Prefixes are checked in a case-insensitive
manner during CURIE expansion.
You can now use an Absolute IRI everywhere you could previously
only use a CURIE (e.g., in the value of
@datatype
).
There is now a concept of a
term
. This concept has
replaced the concept of a 'reserved word'. It is possible now to
use a 'term' in most places where you could previously only use a
CURIE.
You can define a default prefix mapping (via
@vocab
that will be used on undefined terms.
When a triple would include an object literal, and there is no
explicit datatype attribute, the object literal will now be a
'plain literal'. In version 1.0 it would have been an
'XMLLiteral'.
The
@inlist
attribute can be used to instruct the
processor to generate RDF lists with the resources rather than
simple triples.
The effect of
@src
is now identical to
@href
rather than
@about
like in version 1.0.
While this specification strives to be as backward compatible as
possible with [
RDFA-SYNTAX
], the changes above mean that there are
some circumstances where it is possible for different RDF triples to
be output for the same document when processed by an RDFa 1.0
processor vs. an RDFa 1.1 processor. In order to minimize these
differences, a document author can do the following:
Use the XHTML+RDFa 1.0 document type as defined in
RDFA-SYNTAX
].
Place a
@version
attribute with the
value
XHTML+RDFa 1.0
on the
html
element.
If there are places in the document where an object literal
MUST
be an XMLLiteral, use
datatype='rdf:XMLLiteral'
If there are places in the document where an object literal
MUST
be a plain literal, use
datatype=''
If there are places in the document where
@src
is
used, add an
@about
(unless already present) with the
same IRI.
When producing XHTML+RDFa 1.1 documents, it is possible to reduce
the incompatibilities with RDFa 1.0 conforming processors by doing
the following:
DO NOT use the
@vocab
feature.
DO NOT rely upon host language defaults for IRI mappings.
DO NOT use absolute IRIs in place of CURIEs.
Use
@xmlns
AND
@prefix
when declaring prefix mappings.
DO NOT use TERMs on
@datatype
@property
or
@typeof
When using TERMs in
@rel
and
@rev
only use ones defined in [
RDFA-SYNTAX
].
Place a
version
attribute with the
value
XHTML+RDFa 1.0
on the
html
element.
If there are places in the document where an object literal
MUST
be an XMLLiteral, use
datatype='rdf:XMLLiteral'
If there are places in the document where an object literal
MUST
be a plain literal, use
datatype=''
If there are places in the document where
@src
is
used, add an
@about
(unless already present) with the
same IRI.
D.
Acknowledgments
This section is non-normative.
At the time of publication, the active members of the RDFa Working
Group were:
Stéphane Corlosquet, MIND Center for Interdisciplinary Informatics
Ivan Herman,
W3C
Gregg Kellogg (Invited Expert)
Niklas Lindström (Invited Expert)
Shane McCarron, Applied Testing and Technology, Inc. (Invited Expert)
Steven Pemberton, Centre for Mathematics and Computer Science (CWI)
Manu Sporny, Digital Bazaar (Chair, Invited Expert)
E.
References
E.1
Normative references
[HTML-RDFA]
Manu Sporny et al.
HTML+RDFa 1.1 - Second Edition
. 17 March 2015. W3C Recommendation. URL:
[OWL2-OVERVIEW]
W3C OWL Working Group.
OWL 2 Web Ontology Language Document Overview (Second Edition)
. 11 December 2012. W3C Recommendation. URL:
[OWL2-PROFILES]
Boris Motik; Bernardo Cuenca Grau; Ian Horrocks; Zhe Wu; Achille Fokoue.
OWL 2 Web Ontology Language Profiles (Second Edition)
. 11 December 2012. W3C Recommendation. URL:
[OWL2-RDF-BASED-SEMANTICS]
Michael Schneider.
OWL 2 Web Ontology Language RDF-Based Semantics (Second Edition)
. 11 December 2012. W3C Recommendation. URL:
[RDF-SYNTAX-GRAMMAR]
Fabien Gandon; Guus Schreiber.
RDF 1.1 XML Syntax
. 25 February 2014. W3C Recommendation. URL:
[RDF11-MT]
Patrick Hayes; Peter Patel-Schneider.
RDF 1.1 Semantics
. 25 February 2014. W3C Recommendation. URL:
[RFC2119]
S. Bradner.
Key words for use in RFCs to Indicate Requirement Levels
. March 1997. Best Current Practice. URL:
[RFC3987]
M. Duerst; M. Suignard.
Internationalized Resource Identifiers (IRIs)
. January 2005. Proposed Standard. URL:
[XHTML-RDFA]
Shane McCarron.
XHTML+RDFa 1.1 - Third Edition
. 17 March 2015. W3C Recommendation. URL:
[XML-NAMES]
Tim Bray; Dave Hollander; Andrew Layman; Richard Tobin; Henry Thompson et al.
Namespaces in XML 1.0 (Third Edition)
. 8 December 2009. W3C Recommendation. URL:
[XML10-4e]
C. M. Sperberg-McQueen et al.
Extensible Markup Language (XML) 1.0 (Fourth Edition)
. 16 August 2006. W3C Recommendation. URL:
[XMLSCHEMA11-2]
David Peterson; Sandy Gao; Ashok Malhotra; Michael Sperberg-McQueen; Henry Thompson; Paul V. Biron et al.
W3C XML Schema Definition Language (XSD) 1.1 Part 2: Datatypes
. 5 April 2012. W3C Recommendation. URL:
E.2
Informative references
[HTML401]
Dave Raggett; Arnaud Le Hors; Ian Jacobs.
HTML 4.01 Specification
. 24 December 1999. W3C Recommendation. URL:
[MICROFORMATS]
Microformats
. URL:
[QNAMES]
N. Walsh.
Using Qualified Names (QNames) as Identifiers in XML Content
. 17 March, 2004. TAG Finding. URL:
[RDF11-PRIMER]
Guus Schreiber; Yves Raimond.
RDF 1.1 Primer
. 24 June 2014. W3C Note. URL:
[RDF11-TESTCASES]
Gregg Kellogg; Markus Lanthaler.
RDF 1.1 Test Cases
. 25 February 2014. W3C Note. URL:
[RDFA-PRIMER]
Ben Adida; Ivan Herman; Manu Sporny; Mark Birbeck.
RDFa 1.1 Primer - Third Edition
. 17 March 2015. W3C Note. URL:
[RDFA-SYNTAX]
Ben Adida; Mark Birbeck; Shane McCarron; Steven Pemberton et al.
RDFa in XHTML: Syntax and Processing
. 14 October 2008. W3C Recommendation. URL:
[RELAXNG-SCHEMA]
Information technology -- Document Schema Definition Language (DSDL) -- Part 2: Regular-grammar-based validation -- RELAX NG
. ISO/IEC 19757-2:2008. URL:
[SAX]
D. Megginson, et al.
SAX: The Simple API for XML
. May 1998. URL:
[TURTLE]
Eric Prud'hommeaux; Gavin Carothers.
RDF 1.1 Turtle
. 25 February 2014. W3C Recommendation. URL:
[WIDGETS-URI]
Marcos Caceres.
Widget URI scheme
. 13 March 2012. W3C Note. URL:
[XHTML11]
Shane McCarron; Masayasu Ishikawa.
XHTML™ 1.1 - Module-based XHTML - Second Edition
. 23 November 2010. W3C Recommendation. URL:
[XML-EXC-C14N]
John Boyer; Donald Eastlake; Joseph Reagle.
Exclusive XML Canonicalization Version 1.0
. 18 July 2002. W3C Recommendation. URL:
[XML10]
Tim Bray; Jean Paoli; Michael Sperberg-McQueen; Eve Maler; François Yergeau et al.
Extensible Markup Language (XML) 1.0 (Fifth Edition)
. 26 November 2008. W3C Recommendation. URL:
[XMLSCHEMA11-1]
Sandy Gao; Michael Sperberg-McQueen; Henry Thompson; Noah Mendelsohn; David Beech; Murray Maloney.
W3C XML Schema Definition Language (XSD) 1.1 Part 1: Structures
. 5 April 2012. W3C Recommendation. URL:

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