RDFa Core 1.1

4.1 RDFa Processor Conformance

A conforming RDFa Processor must make available to a consuming application a single RDF graph containing all possible triples generated by using the rules in the Processing Model section. This specification uses the term default graph to mean all of the triples asserted by a document according to the Processing Model section.

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

A conforming RDFa Processor must preserve whitespace in both plain literals and XML literals. However, it may be the case that the architecture in which a processor operates does not make all whitespace available. It is therefore advisable for authors who would like to make their documents consumable across different processors, to remove any unnecessary whitespace in their markup.

4.2 RDFa Host Language Conformance

Host Languages that incorporate RDFa must adhere to the following:

• All of the facilities required in this specification must be included in the Host Language.
• The attributes defined in this specification must be included in the content model of the Host Language.
• If the Host Language uses XML Namespaces [XML-NAMES], the attributes in this specification should be incorporated in the namespace of the Host Language.
• 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.

7.1 Overview

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

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

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

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

7.2 Evaluation Context

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

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

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

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

• An initially empty list of URI mappings, called the local list of URI mappings.
• An initially empty list of incomplete triples, called the local list of incomplete triples.
• An initially empty language value.
• A recurse flag. Processing generally continues recursively through the entire tree of elements available. However, if an author indicates that some branch of the tree should be treated as an XML literal, no further processing should take place on that branch, and setting this flag to false would have that effect.
• A skip element flag, which indicates whether the current element can safely be ignored since it has no relevant RDFa attributes. Note that descendant elements will still be processed.
• A new subject value, which once calculated will set the parent subject property in an evaluation context, as well as being used to complete any incomplete triples, as described in the next section.
• A value for the current object literal, the literal to use when creating triples that have a literal object.
• A value for the current object resource, the resource to use when creating triples that have a resource object.
• The local term mappings, a list of terms and their associated URIs.
• A local default vocabulary, a URI to use as a prefix mapping when a term is used.

7.3 Chaining

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

<div about="http://dbpedia.org/resource/Albert_Einstein">
  <span property="foaf:name">Albert Einstein</span>
  <span property="dbp:dateOfBirth" datatype="xsd:date">1879-03-14</span>
  <div rel="dbp:birthPlace" resource="http://dbpedia.org/resource/Germany">
    <span property="dbp:conventionalLongName">Federal Republic of Germany</span>
  </div>
</div>

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

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

<div about="http://dbpedia.org/resource/Albert_Einstein">
  <div rel="dbp:citizenship" resource="http://dbpedia.org/resource/Germany"></div>
  <div rel="dbp:citizenship" resource="http://dbpedia.org/resource/United_States"></div>
</div>

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

<div about="http://dbpedia.org/resource/Albert_Einstein" rel="dbp:citizenship">
  <span about="http://dbpedia.org/resource/Germany"></span>
  <span about="http://dbpedia.org/resource/United_States"></span>
</div>

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

<http://dbpedia.org/resource/Albert_Einstein>
  dbp:citizenship <http://dbpedia.org/resource/Germany> .
<http://dbpedia.org/resource/Albert_Einstein>
  dbp:citizenship <http://dbpedia.org/resource/United_States> .

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:

<div about="http://dbpedia.org/resource/Albert_Einstein">
  <div rel="foaf:depiction">
    <img src="http://en.wikipedia.org/wiki/Image:Albert_Einstein_Head.jpg" />
  </div>
</div>

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

<div about="http://dbpedia.org/resource/Albert_Einstein">
  <div rel="dbp:citizenship">
    <span about="http://dbpedia.org/resource/Germany"></span>
    <span about="http://dbpedia.org/resource/United_States"></span>
  </div>
</div>

7.4 CURIE and URI Processing

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

URI

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

SafeCURIEorCURIEorURI

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

TERMorCURIEorURI

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

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

http://dbpedia.org/resource/Albert_Einstein

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

<div prefix="db: http://dbpedia.org/">
  ...
</div>

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

<div prefix="db: http://dbpedia.org/">
  <div about="db:resource/Albert_Einstein">
    ...
  </div>
</div>

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

<div prefix="dbr: http://dbpedia.org/resource/">
  <div about="dbr:Albert_Einstein">
    ...
  </div>
  <div about="dbr:Baruch_Spinoza">
    ...
  </div>
</div>

<div prefix="dbr: http://dbpedia.org/resource/">
  <div about="dbr:Albert_Einstein">
    ...
  </div>
</div>
<div prefix="dbr: http://someotherdb.org/resource/">
  <div about="dbr:Albert_Einstein">
    ...
  </div>
</div>

<div about="http://dbpedia.org/resource/Albert_Einstein">
  ...
</div>

<div about="dbr:Albert_Einstein">
  ...
</div>

<div about="[dbr:Albert_Einstein]">
  ...
</div>

<div about="[Albert_Einstein]">
  ...
</div>

<div about="Albert_Einstein">
  ...
</div>

term     :=  NCName
    

<link rel="next" href="http://example.org/page2.html" />
<link rel="xhv:next" href="http://example.org/page2.html" />

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

<link about="_:john" rel="foaf:mbox"
  href="mailto:john@example.org" />
<link about="_:sue" rel="foaf:mbox"
  href="mailto:sue@example.org" />
<link about="_:john" rel="foaf:knows"
  resource="_:sue" />

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

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 URIs in meta or link elements even if those elements are before the base element in the stream).

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

• the base is set to the URL 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 URI mappings is empty;
• the list of incomplete triples is empty;
• the language is set to null.
• the term mappings is set to null (or a Host Language defined list).
• the default vocabulary is set to null (or a Host Language defined value).

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.

This specification assumes that certain elements are present in the Host Language (e.g., head). If these elements are not supported in the Host Language, then the corresponding processing rules are not relevant for that language.

The working group as not reached consensus as to whether to include the optional attributes in this specification, or whether to have them defined in the relevant Host Language specifications.

The processing rules are:

1. First, the local values are initialized, as follows:
   • the recurse flag is set to 'true';
   • the skip element flag is set to 'false';
   • new subject is set to null;
   • current object resource is set to null;
   • the local list of URI mappings is set to the list of URI mappings from the evaluation context;
   • the local list of incomplete triples is set to null;
   • the current language value is set to the language value from the evaluation context.
   • the local term mappings is set to the term mappings from the evaluation context.
   • the local default vocabulary is set to the default vocabulary from the evaluation context.
   Note that some of the local variables are temporary containers for values that will be passed to descendant elements via 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.
2. First the current element is examined for any change to the default vocabulary via @vocab. If @vocab is present and contains a value, its value updates the local default vocabulary. If the value is empty, then the local default vocabulary must be reset to the Host Language defined default.
   A Host Language is not required to define a default vocabulary. In such a case, setting @vocab to the empty value has the effect of clearing the local default vocabulary.
3. Next the current element is parsed for any updates to the local term mappings and local list of URI mappings via @profile. If @profile is present, its value is processed as defined in RDFa Profiles.
   Any new terms or URI mappings are merged into respective local lists. They are in effect for this element and for its children.
4. Next, the current element is then examined for URI mappings and these are added to the local list of URI mappings. Note that a URI mapping will simply overwrite any current mapping in the list that has the same name;
   Mappings are defined via @prefix. For backward compatibility, some Host Languages may also permit the definition of mappings via @xmlns. In this case, the value to be mapped is set by the XML namespace prefix, and the value to map is the value of the attribute — a URI. Regardless of how the mapping is declared, the value to be mapped must be converted to lower case, and the URI is not processed in any way; in particular if it is a relative path it is not resolved against the current base. Authors should not use relative paths as the URI.
5. The current element is also parsed for any language information, and if present, current language is set accordingly;
   Host Languages that incorporate RDFa may provide a mechanism for specifying the natural language of an element and its contents (e.g., XML provides the general-purpose XML attribute @xml:lang).
6. If the current element contains no @rel or @rev attribute, then the next step is to establish a value for new subject. Any of the attributes that can carry a resource can set new subject;
   new subject is set to the URI obtained from the first match from the following rules:

   • by using the URI from @about, if present, obtained according to the section on CURIE and URI Processing;
   • otherwise, by using the URI from @src, if present, obtained according to the section on CURIE and URI Processing.
   • otherwise, by using the URI from @resource, if present, obtained according to the section on CURIE and URI Processing;
   • otherwise, by using the URI from @href, if present, obtained according to the section on CURIE and URI Processing.

   If no URI is provided by a resource attribute, then the first match from the following rules will apply:

   • 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';
7. If the current element does contain a @rel or @rev attribute, then the next step is to establish both a value for new subject and a value for current object resource:
   new subject is set to the URI obtained from the first match from the following rules:

   • by using the URI from @about, if present, obtained according to the section on CURIE and URI Processing;
   • otherwise, by using the URI from @src, if present, obtained according to the section on CURIE and URI Processing.

   If no URI is provided then the first match from the following rules will apply:

   • if @typeof is present, then new subject is set to be a newly created bnode;
   • otherwise, if parent object is present, new subject is set to that.

   Then the current object resource is set to the URI obtained from the first match from the following rules:

   • by using the URI from @resource, if present, obtained according to the section on CURIE and URI Processing;
   • otherwise, by using the URI from @href, if present, obtained according to the section on CURIE and URI Processing.

   Note that final value of the current object resource will either be null (from initialization) or a full URI.

8. If in any of the previous steps a new subject was set to a non-null value, it is now used to provide a subject for type values;
   One or more 'types' for the new subject can be set by using @typeof. If present, the attribute must contain one or more URIs, obtained according to the section on URI and CURIE Processing, each of which is used to generate a triple as follows:

   subject

   new subject

   predicate

   http://www.w3.org/1999/02/22-rdf-syntax-ns#type

   object

   full URI of 'type'

   Note that none of this block is executed if there is no new subject value, i.e., new subject remains null.
9. If in any of the previous steps a current object resource was set to a non-null value, it is now used to generate triples:
   Predicates for the current object resource can be set by using one or both of the @rel and @rev attributes:

   • If present, @rel may contain one or more URIs, obtained according to the section on CURIE and URI Processing each of which is used to generate a triple as follows:

     subject

     new subject

     predicate

     full URI

     object

     current object resource

   • If present, @rev may contain one or more URIs, obtained according to the section on CURIE and URI Processing each of which is used to generate a triple as follows:

     subject

     current object resource

     predicate

     full URI

     object

     new subject

10. If however current object resource was set to null, but there are predicates present, then they must be stored as incomplete triples, pending the discovery of a subject that can be used as the object. Also, current object resource should be set to a newly created bnode;
    Predicates for incomplete triples can be set by using one or both of the @rel and @rev attributes:

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

      predicate

      full URI

      direction

      forward

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

      predicate

      full URI

      direction

      reverse

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

    • as a typed literal if:
      • @datatype is present, and does not have an empty value, and is not set to rdf:XMLLiteral.

      The actual literal is either the value of @content (if present) or a string created by concatenating the value of all descendant text nodes, of the current element in turn. The final string includes the datatype URI, as described in [RDF-CONCEPTS], which will have been obtained according to the section on CURIE and URI Processing.

    • as a plain literal if:
      • @content is present;
      • or all children of the current element are text nodes;
      • or there are no child nodes (in which case the literal value is the empty string);
      • or the body of the current element does have non-text child nodes but @datatype is present, with an empty value.

      Additionally, if there is a value for current language then the value of the plain literal should include this language information, as described in [RDF-CONCEPTS]. The actual literal is either the value of @content (if present) or a string created by concatenating the text content of each of the descendant elements of the current element in document order.

    • as an XML literal if:
      • the current element has any child nodes that are not simply text nodes, and @datatype is not present, or is present, but is set to rdf:XMLLiteral.

      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 rdf:XMLLiteral. The format of the resulting serialized content is as defined in Exclusive XML Canonicalization Version [XML-EXC-C14N].

    The current object literal is then used with each predicate to generate a triple as follows:

    subject

    new subject

    predicate

    full URI

    object

    current object literal

    Once the triple has been created, if the datatype of the current object literal is rdf:XMLLiteral, then the recurse flag is set to false.

12. If the skip element flag is 'false', and new subject was set to a non-null value, then any incomplete triples 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 URIs. This list is iterated, and each of the predicates is used with parent subject and new subject to generate a triple. Note that at each level there are two lists of incomplete triples; one for the current processing level (which is passed to each child element in the previous step), and one that was received as part of the evaluation context. It is the latter that is used in processing during this step.
    Note that each incomplete triple has a direction value that it used to determine what will become the subject, and what will become the object, of each generated triple:

    • If direction is 'forward' then the following triple is generated:

      subject

      parent subject

      predicate

      the predicate from the iterated incomplete triple

      object

      new subject

    • If direction is not 'forward' then this is the triple generated:

      subject

      new subject

      predicate

      the predicate from the iterated incomplete triple

      object

      parent subject

13. If the recurse flag is 'true', all elements that are children of the current element are processed using the rules described here, using a new evaluation context, initialized as follows:
    • If the skip element flag is 'true' then the new evaluation context is a copy of the current context that was passed in to this level of processing, with the language and list of URI mappings values replaced with the local values;
    • Otherwise, the values are:
      • the base is set to the base value of the current evaluation context;
      • the parent subject is set to the value of new subject, if non-null, or the value of the parent subject of the current evaluation context;
      • the parent object is set to value of current object resource, if non-null, or the value of new subject, if non-null, or the value of the parent subject of the current evaluation context;
      • the list of URI mappings is set to the local list of URI mappings;
      • the list of incomplete triples is set to the local list of incomplete triples;
      • language is set to the value of current language.
      • the term mappings is set to the value of the local term mappings.
      • the default vocabulary is set to the value of the local default vocabulary.

8.1 Changing the evaluation context

<html>
  <head>
    <title>Jo's Friends and Family Blog</title>
    <link rel="foaf:primaryTopic" href="#bbq" />
    <meta property="dc:creator" content="Jo" />
  </head>
  <body>
    ...
  </body>
</html>

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

<html>
  <head>
    <title>Jo's Blog</title>
  </head>
  <body>
    <h1><span property="dc:creator">Jo</span>'s blog</h1>
    <p>
      Welcome to my blog.
    </p>
  </body>
</html>

<> dc:creator "Jo" .

<html>
  <head>
    <base href="http://www.example.org/jo/blog" />
    <title>Jo's Friends and Family Blog</title>
    <link rel="foaf:primaryTopic" href="#bbq" />
    <meta property="dc:creator" content="Jo" />
  </head>
  <body>
    ...
  </body>
</html>

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

<html>
  <head>
    <title>Jo's Friends and Family Blog</title>
    <link rel="foaf:primaryTopic" href="#bbq" />
    <meta property="dc:creator" content="Jo" />
  </head>
  <body>
    <p about="#bbq" typeof="cal:Vevent">
      I'm holding
      <span property="cal:summary">
        one last summer barbecue
      </span>,
      on
      <span property="cal:dtstart" content="2015-09-16T16:00:00-05:00" 
            datatype="xsd:dateTime">
        September 16th at 4pm
      </span>.
    </p>
  </body>
</html>

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

John knows
<a about="mailto:john@example.org"
  rel="foaf:knows" href="mailto:sue@example.org">Sue</a>.

Sue knows
<a about="mailto:sue@example.org"
  rel="foaf:knows" href="mailto:jim@example.org">Jim</a>.

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

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

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

<img src="photo1.jpg" rel="license" 
     resource="http://creativecommons.org/licenses/by/2.0/" />

<img src="photo1.jpg"
  rel="license" resource="http://creativecommons.org/licenses/by/2.0/"
  property="dc:creator" content="Mark Birbeck"
/>

<div about="http://www.blogger.com/profile/1109404" rel="foaf:img">
  <img src="photo1.jpg"
    rel="license" resource="http://creativecommons.org/licenses/by/2.0/"
    property="dc:creator" content="Mark Birbeck"
  />
</div>

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

<div typeof="foaf:Person">
  <span property="foaf:name">Albert Einstein</span>
  <span property="foaf:givenname">Albert</span>
</div>

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

<div about="http://dbpedia.org/resource/Albert_Einstein">
  <span property="foaf:name">Albert Einstein</span>
  <span property="dbp:dateOfBirth" datatype="xsd:date">1879-03-14</span>
  <div rel="dbp:birthPlace" resource="http://dbpedia.org/resource/Germany" />
  <span about="http://dbpedia.org/resource/Germany"
    property="dbp:conventionalLongName">Federal Republic of Germany</span>
</div>

<div about="http://dbpedia.org/resource/Albert_Einstein">
  <span property="foaf:name">Albert Einstein</span>
  <span property="dbp:dateOfBirth" datatype="xsd:date">1879-03-14</span>
  <div rel="dbp:birthPlace">
    <span about="http://dbpedia.org/resource/Germany"
      property="dbp:conventionalLongName">Federal Republic of Germany</span>
  </div>
</div>

<div about="http://dbpedia.org/resource/Albert_Einstein">
  <span property="foaf:name">Albert Einstein</span>
  <span property="dbp:dateOfBirth" datatype="xsd:date">1879-03-14</span>
  <div rel="dbp:birthPlace" resource="http://dbpedia.org/resource/Germany">
    <span property="dbp:conventionalLongName">Federal Republic of Germany</span>
  </div>
</div>

<div about="http://dbpedia.org/resource/Albert_Einstein">
  <span property="foaf:name">Albert Einstein</span>
  <span property="dbp:dateOfBirth" datatype="xsd:date">1879-03-14</span>
  <div rel="dbp:birthPlace" resource="http://dbpedia.org/resource/Germany">
    <span property="dbp:conventionalLongName">Federal Republic of Germany</span>
    <span rel="dbp:capital" resource="http://dbpedia.org/resource/Berlin" />
  </div>
</div>

<http://dbpedia.org/resource/Albert_Einstein> foaf:name "Albert Einstein" .
<http://dbpedia.org/resource/Albert_Einstein> dbp:dateOfBirth "1879-03-14"^^xsd:date .
<http://dbpedia.org/resource/Albert_Einstein> dbp:birthPlace <http://dbpedia.org/resource/Germany> .

<http://dbpedia.org/resource/Germany>
  dbp:conventionalLongName "Federal Republic of Germany" .
<http://dbpedia.org/resource/Germany>
  dbp:capital <http://dbpedia.org/resource/Berlin> .

<div about="http://dbpedia.org/resource/Baruch_Spinoza" rel="dbp:influenced">
  <div about="http://dbpedia.org/resource/Albert_Einstein">
    <span property="foaf:name">Albert Einstein</span>
    <span property="dbp:dateOfBirth" datatype="xsd:date">1879-03-14</span>
  </div>
</div>

<http://dbpedia.org/resource/Baruch_Spinoza>
  dbp:influenced <http://dbpedia.org/resource/Albert_Einstein> .
<http://dbpedia.org/resource/Albert_Einstein> foaf:name "Albert Einstein" .
<http://dbpedia.org/resource/Albert_Einstein> dbp:dateOfBirth "1879-03-14"^^xsd:date .

<div about="http://dbpedia.org/resource/Baruch_Spinoza" rel="dbp:influenced">
  <div>
    <span property="foaf:name">Albert Einstein</span>
    <span property="dbp:dateOfBirth" datatype="xsd:date">1879-03-14</span>
  </div>
</div>

<http://dbpedia.org/resource/Baruch_Spinoza> dbp:influenced _:a .
_:a foaf:name "Albert Einstein" .
_:a dbp:dateOfBirth "1879-03-14"^^xsd:date .

<div about="http://dbpedia.org/resource/Baruch_Spinoza" rel="dbp:influenced">
  <span property="foaf:name">Albert Einstein</span>
  <span property="dbp:dateOfBirth" datatype="xsd:date">1879-03-14</span>
</div>

<div about="http://dbpedia.org/resource/Baruch_Spinoza">
  <div rel="dbp:influenced">
    <span property="foaf:name">Albert Einstein</span>
    <span property="dbp:dateOfBirth" datatype="xsd:date">1879-03-14</span>
  </div>
</div>

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 Germany:

<div about="http://dbpedia.org/resource/Albert_Einstein">
  <span property="foaf:name">Albert Einstein</span>
  <span property="dbp:dateOfBirth" datatype="xsd:date">1879-03-14</span>
  <div rel="dbp:birthPlace" resource="http://dbpedia.org/resource/Germany" />
</div>

and then a further statement that the 'long name' for Germany is the Federal Republic of Germany:

<span about="http://dbpedia.org/resource/Germany"
  property="dbp:conventionalLongName">Federal Republic of Germany</span>

RDFa allows authors to insert this statement as a self-contained unit into other contexts:

<div about="http://dbpedia.org/resource/Albert_Einstein">
  <span property="foaf:name">Albert Einstein</span>
  <span property="dbp:dateOfBirth" datatype="xsd:date">1879-03-14</span>
  <div rel="dbp:birthPlace" resource="http://dbpedia.org/resource/Germany" />
  <span about="http://dbpedia.org/resource/Germany"
    property="dbp:conventionalLongName">Federal Republic of Germany</span>
</div>

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

<div about="http://dbpedia.org/resource/Albert_Einstein">
  <span property="foaf:name">Albert Einstein</span>
  <span property="dbp:dateOfBirth" datatype="xsd:date">1879-03-14</span>
  <div rel="dbp:birthPlace">
    <span about="http://dbpedia.org/resource/Germany"
      property="dbp:conventionalLongName">Federal Republic of Germany</span>
  </div>
</div>

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:

<http://dbpedia.org/resource/Albert_Einstein> dbp:birthPlace ? .

Then as processing continues, the RDFa Processor encounters the subject of the statement about the long name for Germany, and this is used in two ways. First it is used to complete the 'incomplete triple':

<http://dbpedia.org/resource/Albert_Einstein> dbp:birthPlace <http://dbpedia.org/resource/Germany> .

and second it is used to generate its own triple:

<http://dbpedia.org/resource/Germany> dbp:conventionalLongName "Federal Republic of Germany" .

Note that each occurrence of @about will complete any incomplete triples. For example, to mark up the fact that Albert Einstein had both German and American citizenship, an author need only specify one @rel value that is then used with multiple @about values:

<div about="http://dbpedia.org/resource/Albert_Einstein" rel="dbp:citizenship">
  <span about="http://dbpedia.org/resource/Germany" />
  <span about="http://dbpedia.org/resource/United_States" />
</div>

In this example there is one incomplete triple:

<http://dbpedia.org/resource/Albert_Einstein> dbp:citizenship ? .

When the processor meets each of the @about values, this triple is completed, giving:

<http://dbpedia.org/resource/Albert_Einstein>
  dbp:citizenship <http://dbpedia.org/resource/Germany> .
<http://dbpedia.org/resource/Albert_Einstein>
  dbp:citizenship <http://dbpedia.org/resource/United_States> .

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 illustrate, to indicate that Spinoza influenced both Einstein and Schopenhauer, the following markup could be used:

<div about="http://dbpedia.org/resource/Baruch_Spinoza">
  <div rel="dbp:influenced">
    <div typeof="foaf:Person">
      <span property="foaf:name">Albert Einstein</span>
      <span property="dbp:dateOfBirth" datatype="xsd:date">1879-03-14</span>
    </div>
    <div typeof="foaf:Person">
      <span property="foaf:name">Arthur Schopenhauer</span>
      <span property="dbp:dateOfBirth" datatype="xsd:date">1788-02-22</span>
    </div>          
  </div>
</div>

First the following incomplete triple is stored:

<http://dbpedia.org/resource/Baruch_Spinoza> dbp: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:

<http://dbpedia.org/resource/Baruch_Spinoza"> dbp:influenced _:a .
_:a rdf:type foaf:Person .
_:a foaf:name "Albert Einstein" .
_:a dbp:dateOfBirth "1879-03-14"^^xsd:date .
<http://dbpedia.org/resource/Baruch_Spinoza"> dbp: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:

<div about="http://dbpedia.org/resource/Baruch_Spinoza" rel="dbp:influenced">
  <span property="foaf:name">Albert Einstein</span>
  <span property="dbp:dateOfBirth" datatype="xsd:date">1879-03-14</span>
  <div rel="dbp:citizenship">
    <span about="http://dbpedia.org/resource/Germany" />
    <span about="http://dbpedia.org/resource/United_States" />
  </div>
</div>

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:

<http://dbpedia.org/resource/Baruch_Spinoza> dbp:influenced ? .

Next, the RDFa Processor processes the predicate values for foaf:name, dbp:dateOfBirth and dbp:citizenship, but note that only the first needs to 'complete' the 'hanging rel'. So processing foaf:name generates two triples:

<http://dbpedia.org/resource/Baruch_Spinoza> dbp:influenced _:a .
_:a foaf:name "Alber Einstein" .

but processing dbp:dateOfBirth generates only one:

_:a dbp:dateOfBirth "1879-03-14"^^xsd:date .

Processing dbp:citizenship also uses the same bnode, but note that it also generates its own 'incomplete triple':

_:a dbp:citizenship ? .

As before, the two occurrences of @about complete the 'incomplete triple', once each:

_:a dbp:citizenship <http://dbpedia.org/resource/Germany> .
_:a dbp:citizenship <http://dbpedia.org/resource/United_States> .

The entire set of triples that an RDFa Processor should generate are as follows:

<http://dbpedia.org/resource/Baruch_Spinoza> dbp:influenced _:a .
_:a foaf:name "Alber Einstein" .
_:a dbp:dateOfBirth "1879-03-14"^^xsd:date .
_:a dbp:citizenship <http://dbpedia.org/resource/Germany> .
_:a dbp:citizenship <http://dbpedia.org/resource/United_States> .

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, URI resources and literals.

A literal object can be set by using @property to express a predicate, and then using either @content, or the inline text of the element that @property is on. 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.

A URI 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.

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

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

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

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

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

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

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

<span property="cal:dtstart" content="2015-09-16T16:00:00-05:00" 
      datatype="xsd:dateTime">
  September 16th at 4pm
</span>.

<> cal:dtstart "2015-09-16T16:00:00-05:00"^^xsd:dateTime .

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

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

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

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

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

<p>You searched for <strong>Einstein</strong>:</p>
<p about="http://dbpedia.org/resource/Albert_Einstein">
  <span property="foaf:name" datatype="">Albert <strong>Einstein</strong></span>
  (b. March 14, 1879, d. April 18, 1955) was a German-born theoretical physicist.
</p>

<http://dbpedia.org/resource/Albert_Einstein> foaf:name "Albert Einstein" .

<h2 property="dc:title">
  Example 3: <span about="#bbq" typeof="cal:Vevent">...</span>
</h2>

<html>
  <head>
    <title>On Crime and Punishment</title>
    <base href="http://www.example.com/candp.xhtml" />
  </head>
  <body>
    <blockquote about="#q1" rel="dc:source" resource="urn:ISBN:0140449132" >
      <p id="q1">
        Rodion Romanovitch! My dear friend! If you go on in this way
        you will go mad, I am positive! Drink, pray, if only a few drops!
      </p>
    </blockquote>
  </body>
</html>

<http://www.example.com/candp.xhtml#q1>
  <http://purl.org/dc/elements/1.1/source> <urn:ISBN:0140449132> .

<link about="mailto:john@example.org"
      rel="foaf:knows" href="mailto:sue@example.org" />

<img src="photo1.jpg" rel="dc:creator" rev="foaf:img"
   href="http://www.blogger.com/profile/1109404" />

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