We recently had a related discussion on the telecon about pre-defining
the top-10 vocabularies in RDFa 1.1 as displayed at prefix.cc. Parsers
could then use common prefixes without relying on xmlns: to define them.
If this sounds like a discussion we had two years ago, it is - the
reason to reconsider it is that people really seem to want this feature.
It would, in effect, be hard-coding these values into the specification.
New specs could be revised to include new prefixes, so references to
vocab="rdfa-2012" and vocab="rdfa-2014" would add new mappings, but
never override old mappings. Thus, this would always be the mapping for