> A variation on the theme would be to descibe it in terms of deltas from
> some preset DTD (W3C's ?) as maintained at some repository. Maintaining
> the DTDs as diffs from the base version.
It would be nice if all the variations of web browser capabilities
could be described as variations in the DTD, but unfortunately, this
isn't the case. For example, for a while, there were some web browsers
that could handle "mailto:" URLs, and others that couldn't. This might
be a plug-in capability, or might be intrinisic to the browser.
While it's a great idea to have a DTD for the HTML capabilities of a
given browser, it isn't sufficient.