> Introducing the concept of an external resource that has to be fetched doesn't seem impossible to me, but there are a lot of issues that would have to be thought through.

I agree that an external resource is a lot of hurdles. What I had in mind while writing the reply was a stripped-down version of TrueType, with only a single `glyf` table allowed (probably with a minimal version of the `head` table as well). Judging from how icon fonts are used today, this might even be possible without a persistent font state.