My own focus is on PDF files. The PDF spec says that fonts declared as
OpenType should be OpenType, and the files I'm looking at violate the
spec. (PDF also supports standalone CFF fonts, so it's relatively
little extra work for me to pull the CFF blob out of these bogus
"OpenType" fonts, and handle it as a CFF font.)
Oh yes, you're right there. However, this is a very general problem. Even though OpenType is a Microsoft trademark, Microsoft had done a very poor job in explaining/defining what is and what ISN'T an "OpenType font".
Windows had been announcing OpenType fonts with a glyf table but without a DSIG table as "TrueType" for many years, even though TrueType is an Apple trademark. So many end-users for a long time associated CFF-based OpenType with "OpenType".
And PDF has its own notion of font format "branding", and then there's a question of how certain apps like Acrobat present these.
I lost count of the different variants of fonts that "can exist" in a PDF a log time ago. I guess a Type42 with a variable CFF2 table is also theoretically possible :) Or with just CBDT, without glyf and CFFx.
One thing for sure: on "desktop", certain fonts work out they don't, but they need to be kind of "complete". But PDF has this flurry of "partial font resources" which makes it extra-complex.
A.