Long standing problem with Vedic tone markers and post-base visarga/anusvara

[Richard Wordingham via Unicode]

On Thu, 2 Jan 2020 15:07:04 -0800
Norbert Lindenberg <norbert at lindenbergsoftware.com> wrote:

>> On Jan 2, 2020, at 12:20, Richard Wordingham via Unicode
>> <unicode at unicode.org> wrote:  

>> So, the problem should already be solved for Grantha, and,
>> if the plans come to fruition, will work with a font whose
>> Devanagari script tag is 'dev3'.  However, I may have overlooked a
>> set of overrides to the USE categorisations.  

> You can create Indic 3 fonts that get processed by the USE today, and
> use them with Harfbuzz (Chrome, Firefox, Android, …) and with
> CoreText (Apple platforms). I don’t know if anybody has already
> created such fonts.
> https://lindenbergsoftware.com/en/notes/brahmic-script-support-in-opentype/

Is there a script tag registry, or is it now a free-for-all as with font
names?  (I suppose it is implicitly constrained by what the individual
renderers recognise.)

The nearest to a registry I can find is at
https://docs.microsoft.com/en-us/typography/opentype/spec/ttoreg, but
that appears to be limited to what Microsoft supports - "The tag
registry defines the OpenType Layout tags that Microsoft supports".
None of the Indic 3 script tags are there.

Richard.