As of 2024-04-10, I intend to remove automatic vertical centering of
some basic binary operators for MathML. This was never implemented in
other browsers and is not described in any specification.
Bug to remove: https://bugzilla.mozilla.org/show_bug.cgi?id=1890531
To understand better, here is a basic example:
<!DOCTYPE html>
<link
rel="stylesheet"
href="https://fred-wang.github.io/MathFonts/STIX/mathfonts.css"; />
<math style="font: 20pt 'STIX Two Math'">
<mfrac>
<mn>12</mn>
<mn>34</mn>
</mfrac>
<mo>+</mo>
<mn>56</mn>
</math>
The fraction bar is placed on the "math axis", which is slightly shifted
away from the baseline. For fonts with an OpenType MATH table, this
shift is given by the AxisHeight parameter. In practice, math fonts are
also designed so that binary operators (e.g. the plus sign) has its
vertical center aligned with that math axis too.
Gecko's MathML implementation always adjusts vertical alignment of a few
basic binary operators (namely +, =, *, − ≤ ≥ and ×)
so that there center is aligned with the math axis. In practice, this is
only necessary when non-math fonts are used for MathML rendering but in
that you'll get many other rendering issues.
Also for the sake of testing, some WPT tests use special web fonts with
intentionally large AxisHeight (and so math axis not aligned with the
middle of the binary operators). Having the automatic adjustment makes
harder to deal with these tests.
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