The RDFa approach

The approach taken by RDFa is that ultimately any RDF structure should be representable. This means that instead of having to 'codify' each format to describe how it must be marked up, we simply provide a set of rules that explain how anyRDF can be marked up, and than anyRDF 'language' can be used.

This means that a library, for example, can still make use of the complex taxonomies and schemas that it relies on, whilst a web author can mark-up their home page using something like Friend-of-a-Friend (FoaF).