Members of the W3C RDFa Working Group have published a First Public Working Draft for the specification "RDFa Core 1.1: Syntax and Processing Rules for Embedding RDF Through Attributes."  The document is intended to become a W3C Recommendation. A sample test harness is available, though its set of tests is not intended to be exhaustive.

Users may find the tests to be useful examples of RDFa usage.  This document is expected to supersede the 'RDFa in XHTML (RDFa 1.0)' specification.

RDFa provides a set of XHTML attributes to augment visual data with machine-readable hints as well as providing a few new ones. Attributes that already exist in widely deployed languages (e.g., HTML) have the same meaning they always did, although their syntax has been slightly modified in some cases. For example, in (X)HTML, '@rel' already defines the relationship between one document and another. However, in (X)HTML there is no clear way to add new values; RDFa sets out to explicitly solve this problem, and does so by allowing URIs as values. It also introduces the idea of 'compact URIs' -- referred to as CURIEs in this document -- which allow a full URI value to be expressed succinctly...

Background: "The current Web is primarily made up of an enormous number of documents that have been created using HTML. These documents contain significant amounts of structured data, which is largely unavailable to tools and applications. When publishers can express this data more completely, and when tools can read it, a new world of user functionality becomes available, letting users transfer structured data between applications and web sites, and allowing browsing applications to improve the user experience: an event on a web page can be directly imported into a user's desktop calendar; a license on a document can be detected so that users can be informed of their rights automatically; a photo's creator, camera setting information, resolution, location and topic can be published as easily as the original photo itself, enabling structured search and sharing.

RDFa Core is a specification for attributes to express structured data in any markup language. The embedded data already available in the markup language (e.g., XHTML) is reused by the RDFa markup, so that publishers don't need to repeat significant data in the document content. The underlying abstract representation is RDF, which lets publishers build their own vocabulary, extend others, and evolve their vocabulary with maximal interoperability over time. The expressed structure is closely tied to the data, so that rendered data can be copied and pasted along with its relevant structure...

RDFa shares some of the same goals with microformats. Whereas microformats specify both a syntax for embedding structured data into HTML documents and a vocabulary of specific terms for each microformat, RDFa specifies only a syntax and relies on independent specification of terms (often called vocabularies or taxonomies) by others. RDFa allows terms from multiple independently-developed vocabularies to be freely intermixed and is designed such that the language can be parsed without knowledge of the specific vocabulary being used..."

http://www.w3.org/TR/2010/WD-rdfa-core-20100422/

See also W3C Semantic Web: http://www.w3.org/standards/semanticweb/

Ben Adida, Mark Birbeck, Shane McCarron, Ivan Herman (eds), W3C TR