Intent to unship: Adjustment of default MathML operator lspace/rspace when math-depth > 0

[Frédéric Wang]

As of Firefox 150, I intend to remove adjustment of default MathML
operator lspace/rspace when math-depth > 0. Status in other browsers is
not implemented.

Bug to remove: https://bugzilla.mozilla.org/show_bug.cgi?id=2018406

Consider the following case:

<math>
  <msup>
    <mi>A</mi>
<mrow><mn>1</mn><mo>+</mo><mn>2</mn></mrow>
  </msup>
</math>

MathML Core defines a default spacing around the plus operator, but
Gecko departs from the spec and only takes half of the spacing because
the mrow is placed as a superscript:
https://searchfox.org/firefox-main/rev/35009c6b3c3a10d189dd24790b5aaa6a27b6058f/layout/mathml/nsMathMLmoFrame.cpp#380

This is some legacy arbitrary tuning from the 1999 implementation but is
not defined in any spec and never implemented in other browsers. The
difference may not be very obvious at the naked eye, in the example
above spacing around the plus is supposed to be 0.2222222222222222em on
each side per the spec while Gecko would currently use
0.1111111111111111em instead ; and additionally the font-size is scaled
down in scripts, so the absolute difference is even smaller.