Considerations for "desiccated" initial font files

Apologies if this is handled somewhere in the spec. I don't remember it being discussed explicitly but maybe it's all implicit.

I know we've discussed the possibility of effectively removing a round-trip by loading the initial font file in CSS, so that it functions analogously to the unicode mapping for unicode-range. I believe this could be done with a Base64-encoded data URL, and maybe it could be done in other ways.

Suppose (because you're paying a penalty for the Base64) you want to minimize the size of the initial file, downloading most of the "universal" data via an initial patch. So let's assume we're doing some combination of per-table patches and glyph-keyed patches (one or the other or both).

• What is the minimum initial table set assuming per-table patches? My previous understanding is that it would be cmap, IFT, and IFTX (when present) but maybe things have expanded since then? Are there certain tables that should be required in order to solve chicken-and-egg problems on the client side -- perhaps "head" or "post"?
• For glyph-keyed patches the normal presumption is that the glyf/loca/GVAR or CFF/CFF2 tables will be present. Maybe this is fine, they should compress pretty well when "empty". If we want that not to be the case, though, and to download those tables in the initial per-table patch, can we have some semantic for doing that without an extra round-trip? (That is, to start downloading the initial per-table patch and the glyph-keyed patches at the same time, but guarantee that the per-table patch is applied first.)
• Should we add language to the effect of "an initial font not yet checked for some set of code points cannot be judged for completeness or OpenType/OFF spec compliance."?

Start by reviewing the Incremental Font Transfer specification and the referenced OpenType/OFF requirements. Resolve the minimum initial table set, patch ordering semantics, and completeness/compliance wording; the issue is done when these decisions are incorporated into the specification.