Created attachment 227948 [details]
Currently, Font::glyphDataForCharacter accepts a mirror boolean to apply Character-level mirroring. However it does not allow to apply Glyph-level mirroring. See
This is needed for MathML operators such as integrals or sums, that don't have character-level mirroring defined in Unicode.
In Blink & Gecko with the XITS fonts installed, the attached testcase shows mirroring of the bracket (character-level) and sum (glyph-level). The latter relies on the 'rtlm' feature of the XITS. Note that for MathML purpose, all what is needed is a way for Font::glyphDataForCharacter to use the rtlm substitution table, not to implement the font-feature-settings CSS property.