HTML and CSS Reference
In-Depth Information
In contrast to microformats and microdata, RDFa is a complete serialization of RDF and hence provides the most
advanced annotation of them all. RDFa defines new markup attributes and uses URIs and namespaces by default.
Consequently, combining vocabularies is very easy (similar to RDF). RDFa is completely flexible regarding literals and
URI resources.
RDF Schema
According to the W3C Metadata Activity,
RDF Schema
(
RDFS
) is “a declarative representation language influenced by
ideas from knowledge representation” [143]. RDF Schema extends RDF with structure (classes, properties of properties,
and so on). It can be used to formalize metadata exchange between human-readable and machine-processable
vocabularies. Beyond the basic RDF vocabulary discussed earlier, RDFS has several additional constructs [144]:
•
Classes
•
rdf:Property
•
rdf:XMLLiteral
•
rdfs:Class
•
rdfs:Datatype
•
rdfs:Literal
•
rdfs:Resource
Properties
•
•
rdf:type
•
rdfs:comment
•
rdfs:domain
•
rdfs:isDefinedBy
•
rdfs:label
•
rdfs:range
•
rdfs:seeAlso
•
rdfs:subClassOf
•
rdfs:subPropertyOf
These classes and properties provide an even more advanced level of knowledge representation than RDF does
and can be used for basic description of web ontologies. This is the reason why a more expressive language, the Web
Ontology Language (OWL), reuses many RDFS components (see next section).
For example, the resource “macaw” can be declared as a subclass of the class “birds,” as shown in Listing 7-69.
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