Private-use metric tags should be in all caps

[dscorbett]

The private-use tags added in #1867 don’t follow the convention for private-use tags: “Private-use tags must use begin with an uppercase letter and use only uppercase letters or digits.”

[behdad]

Note that I also added a private style tag in 39c20d2

we should apply whatever we conclude to that as well. That one also uses one capital letter (last one).

[behdad]

Now to my main argument. I believe this is correct as is. The OT spec uses all-lowercase tags. It reserves all-uppercase tags for font vendors. Anything in-between is for system-developers, like HarfBuzz.

Originally I had added Harf/Buzz OT features as well. I wish I had not changed that (upon suggestion) to HARF/BUZZ in a62f37d

[behdad]

Note that other systems, like C/C++ are the same: C/C++ language say any identifier starting with __ are reserved. Those are used by system libraries though. The "reservation" does not apply to the implementation of the language itself. Similarly, the reservation doesn't apply to HarfBuzz, which is an implementation of OpenType.

[dscorbett]

Hmm, I guess that makes sense. I wish the spec would make that interpretation more explicit though.

[behdad]

Okay I re-added Harf and Buzz features, considering them required; whereas I see the HARF and BUZZ features as discretionary (user can turn on/off). So they serve distinct purposes now. Might need documentation update.

[behdad]

Good to close?

[behdad]

Note that other systems, like C/C++ are the same: C/C++ language say any identifier starting with __ are reserved. Those are used by system libraries though. The "reservation" does not apply to the implementation of the language itself. Similarly, the reservation doesn't apply to HarfBuzz, which is an implementation of OpenType.

This interpretation is also in line with Graphite's use of Silf etc. tags.