You could say that this is what Microformats are trying to achieve. The problem that Microformats has at the moment is that there isn't enough developers implementing them to make anything particularly useful, which in turn puts off other developers from implementing them as they think "what's the point?". Hopefully in time, Microformats reach a tipping point whereby it's the next hip thing a la Ajax and Rails.
Actually, what I'd like to see is web-wide tagging and then let the tags evolve. For example, I'd like to be able to wrap anything in a web page with <tag name="?"></tag>
The main difference between your idea and eRDF or RDFa is lack of namespaces. The "web-wide tagging" and "let the tagging evolve" bits are a huge part of RDF.
