RDFa - Resource Description Framework - in - attributes

5 Principles of interoperable metadata:

• Publisher Independence - each site can use its own standards
• Data Reuse - data is not duplicated. You do not need separate XML and HTML sections for the same content.
• Self Containment - The HTML and the RDF are separated
• Schema Modularity - The attributes are reusable
• Evolvability - additional fields can be added and XML transforms can extract the semantics of the data from an XHTML file

RDF: http://en.wikipedia.org/wiki/Resource_Description_Framework

RDFa: http://en.wikipedia.org/wiki/RDFa