This was one of the use cases some of us had in mind when RDFa was being developed, and it is nice to see that happening in practice… Olaf Hartig and Jun Zhao have recently published a provenance vocabulary. I am not knowledgeable enough to get into the detail of the provenance part. However, what also caught my attention is the way the vocabulary is defined: it is using XHTML+RDFa. So, while the URI above leads to a nice XHTML version of the vocabulary, readable by humans, the same source can also be used to get to the formal, RDF version of the vocabulary. Just use a distiller or extractor of any kind. I.e., do not repeat yourself, even when defining a formal vocabulary… I find this cool.