Bug 281206 – Disable some legacy MathML attributes and values on all ports

• Component: MathML
• Status: NEW

[Frédéric Wang (:fredw)]

The following legacy features are implemented in WebKit but are not available in any other browser for a while:

- numalign/denomalign attributes (*)
- mfrac's linethickness values: thin, medium, thick values (*)
- nonzero unitless values (e.g. 5 to mean 500%) for MathML lengths (*)
- subscriptshift/superscriptshift attributes.
- mstyle attributes: background, color, fontsize, fontstyle, fontweight
- mathsize values: small, normal, big (*)

The features marked as (*) can be disabled via https://searchfox.org/wubkat/rev/168430191bc069c153b23fdddfc818adea1f2992/Source/WTF/Scripts/Preferences/UnifiedWebPreferences.yaml#2062. On WPE/GTK, we already disable them by default but we might consider doing that for all ports. Be aware though that this preference also disables more things such as named spaces (see https://searchfox.org/wubkat/rev/168430191bc069c153b23fdddfc818adea1f2992/Source/WebCore/mathml/MathMLPresentationElement.cpp#197).

Some WPT tests (probably non-exhaustive):

  - https://wpt.fyi/results/mathml/presentation-markup/fractions/frac-numalign-denomalign-001.html
  - https://wpt.fyi/results/mathml/presentation-markup/fractions/frac-linethickness-001.html
  - https://wpt.fyi/results/mathml/relations/css-styling/lengths-1.html
  - https://wpt.fyi/results/mathml/relations/css-styling/lengths-2.html
  - https://wpt.fyi/results/mathml/presentation-markup/scripts/subsup-legacy-scriptshift-attributes-001.tentative.html
  - https://wpt.fyi/results/mathml/relations/css-styling/attribute-mapping-001.html

[Ahmad Saleem]

I tried this yesterday as well:

- mathsize values: small, normal, big (*)


It compiles and do make us progress `length-1.html` test and it is relatively easy to do.

[Radar WebKit Bug Importer]

<rdar://problem/138098857>