Define that (and how) glyph assemblies are mirrored in rtl formulas

[bert-github]

(This is part of the I18n WG review.)

B.4 Unicode-based Glyph Assemblies
https://www.w3.org/TR/mathml-core/#unicode-based-glyph-assemblies

Maybe this section should say explicitly that the glyph assemblies are mirrored in right-to-left formulas. Firefox and Safari mirror them already, Chromium doesn't (but maybe that's an implementation bug).

Unicode defines for some symbols what their mirrored versions are, but it doesn't do so for these ‘extension’ characters. So maybe the MathML Core spec itself should list the mirrored glyphs. It could add them as an extra column in the table.

(The Unicode Technical Committee is still looking into whether to define some of these extension symbols as mirrored in a future version of Unicode, based on a larger proposal from 2022 (‘Glyph mirroring: ExtraMirroring.txt’).)

[fred-wang]

This section not implemented in Chromium (it is non-normative and is only a fallback mechanism for a few operators).

The general mechanism would be to do something like #67 (comment) which is what Firefox implements. However, that was discarded from MathML Core level 1 and from the initial Chromium release.