Hmm... what about using something like the spot/span tags to do this:
<span class=index id=ix1 end=eix1>indexed term<spot id=eix1>
This could work for indexing terms, search keywords, and several
other applications.
On the other hand, indexing markup is a pretty widely used
application, and perhaps it merits its own tag. It depends on whether
you expect folks to key in documents by hand, or whether you expect
this sort of stuff to be spit out by stuff like WebMaker.
If you expect the markup for your document to be concise, then the DTD
should be crafted around the type of document you're building -- you
might want to craft an RFC DTD, for example. But if you're not worried
about verbose markup, the spot/span idioms are good for all sorts of
applications.
For idioms that are essentially long-standing institutions (TOC,
index, bibliography/references) I don't mind a dedicated tag in
HTML. But for anything even _remotely_ application specific (like
strike-out and change bars for legal documents), I'd rather use a more
generic tag in combination with CLASS.
Dan