[MathML 4] Add rules to map from non-combining to combining accents

Hello,

Another idea for a future MathML spec.

MathML suggests to use non-combining characters for operators and most
of the operators in its dictionary are non-combining. However, TeX seems
to rely on combining characters for accents and hence in practice
OpenType fonts with a MATH table only provide size variants or glyph
assemblies for these combining characters ; and nothing for the
non-combining equivalents. That means non-combining characters are not
stretchable with these fonts and users may be tempted to use the
combining versions instead.

I think it would be good if MathML specifies how web engines can
fallback from non-combining to combining accents in order to stretch
accents. Ideally, we should have a list of such mappings (maybe data in
the XML Entity Definitions for Characters). For example to stretch
U+00AFMACRON, WebKit will also try and find size variants or glyph
assemblies associated to that character inthe OpenType MATH table and
otherwise fallback to data associated to U+0304 (COMBINING MACRON) or
U+0305 (COMBINING OVERLINE).

Having such a list will allow to improve consistencies between web
engines and to write WPT tests for that feature (currently it's
non-standard so only tested in WebKit repo).

WebKit bug: https://bugs.webkit.org/show_bug.cgi?id=159513

Mozilla bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1285277

-- 
Frédéric Wang - frederic-wang.fr