<p ctag="wikipedia/The_Beatles">We're talking about The Beatles here</p>
It even seems like anything that could be tagged should be well-formed XML document (well, XHTML).
What is needed are overlays that sit on top of the content. This way, you can specify tags for whatever sections you want, and I can do the same while the content remains untouched. The advantage is you can share your overlays with your friends, or you could even setup a centralized service that aggregates the overlays and uses the most common sets of tags.
The overlaid, personal tags could be through some other mechanism that wouldn't even be part of the page. It could be some browser plugin that maps my tags for the band Beatles.
They should be using overlaid personal tags as well, it's just when the webpage is served, their tags would be sent over as well. Then you can choose if you want to use their tags or your own personal ones.
Or more like Ted Nelson's idea, "Embedded Markup Considered Harmful"?
Just out of curiosity - why would you want to tag somebody's content?
You lack imagination! :P Tagging = annotation ;-) why wouldn't you want to be able to add some form of notes/annotations to some document??