I'd include lists and tables for data presentation. Also, we may want some (additional) tags to manage footnotes and references (these were in SGMLguid, but didn't make it into HTML.)

O, yes, we forgot that. Footnotes, lists, and tables, are good too.

What about img? It's not inconceivable that an article or a reference manual should have images of some kind.

You could just link to them I think, possibly with a hint to specify inlining; the client decides whether to obey that hint or to ignore that hint. (This would be done the same whether the picture is part of the document or is a separate file, I should think. It makes many considerations easier to work with.)

This is actually how it worked in the beginning. Browsers also had an "display images" toggle button in the main control bar.

I guess, the Netscape Navigator 1.0 specs (tables and forms, but before frames and JS) are quite what we may want (with the possible exception of the font tag and, certainly, without the blink tag), augmented by some footnote/reference tags and a reduced set of CSS.