RDFa

Resource Description Framework with attributes(RDFa) is a set of extensions for XHTML. These extensions allow content providers to add metadata as attributes to web content in XHTML documents. The recommendation includes only syntax. Pre-defined context specific vocabularies can be used to define attributes. This allows data currently displayed on web pages to be parsed into machine readable data as RDF triples without the need to understand the complexities of RDF. In contrast to Microformats, displayed data is used without the need to embed replicated data within the document.

The World Wide Web Consortium(W3C) published the 'RDFa recommendation in XHTML: Syntax and Processing' recommendation on October 14, 2008]. Support for RDFa allows web site owners to publish specific data about events, products, services, people and more. That data can be read and understood by search engines and other software. Yahoo, Google and Bing provide enhanced search results for various vocabularies expressed using RDFa. Incorporation of RDFa metadata into web pages is a stepping stone to Tim Berners-Lee's original vision of the web of meaning also referred to as the Semantic Web. It goes beyond the limits of Microformats by allowing the use of all existing vocabularies.

Both RDFa and Microformats render machine readable data. RDFa defines only syntax. Support for attributes comes from a growing number of vocabularies, also known as ontologies. Vocabularies are extensible. Microformats are limited to a small number of specifications. RDFa attributes are added in-line to existing data, while Microformats require embedded syntax and data even if the data is already within the document. RDFa also supports the application of metadata to documents other than the current one.

RDFa validates agains XHTML 1.1 + RDFa DTD. It will not validate against HTML 4.01.