PFE challenges to consider

Folks,

I've been chatting with one of my colleagues (who is the expert in complex scripts) about our progressive font enrichment project, primarily to figure out what fonts we'd need to use as part of our test set for analysis framework. As I explained to him two different approaches we currently consider, and the overall goals of this project, he made a casual remark during the discussion saying "for best results and highest level of efficiency - make sure you are subsetting the font to output glyphs set, and not just based on input data".

This seemingly innocent remark has immediately raised multiple issues we didn't consider yet (or, at least didn't verbalize):
- output glyphs can be modified by CSS (think e.g. stylistic sets, smallcaps, glyph alternates, etc.) - a font subset created to support a particular page has to account for this;
- output glyphs can be modified by a particular rendering mode (e.g. ruby markup in Japanese);
- output glyphs are subject to shaping / layout rules, we may not always know what they are (even if we know all input character combinations) until the shaping is done, which means the first increment of a particular font has to be loaded to at least support shaping.

I am sure there is more to consider, this is just the tip of an iceberg. As is, these considerations seem to create certain additional challenges for incremental transfer, and also give bit more weight to an alternative approach Myles has suggested, when a browser can ask for the basic subset to start with and incrementally update it based on real needs determined by shaping and CSS.

Thoughts?

Thank you,
Vlad