Hi,

The mfrac elements has values "thin", "medium" and "thick" which
are "left up to the rendering agent". This is prone to
inconsistencies between implementations. During Igalia's MathML
refactoring, we aligned WebKit on Gecko's behavior. This is
essentially what is suggested in the MathML in HTML5
implementation note and that should probably be included in an
official MathML recommendation too:

* The default linethickness "medium" is given by the FractionRuleThickness
constant of the MATH table of the font used for the MathML
rendering (see
https://lists.w3.org/Archives/Public/www-math/2016Jul/0025.html
for discussion about how to determine that font).

* "thin" is 50% and
"thick" is 200% of the default linethickness (these values are
arbitrary and based on how it has been implemented in Gecko
probably for more than 15 years).

But are these
keywords really used in practice? If not maybe you may be
inclined to deprecate them too.

Frédéric