As reported in https://github.com/w3c/mathml/issues/75#issuecomment-1273061494 we currently generate named MathML with named length attributes
<math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle a\quad \,\;\!\qquad b}">
<semantics>
<mrow class="MJX-TeXAtom-ORD">
<mstyle displaystyle="true" scriptlevel="0">
<mi>a</mi>
<mspace width="1em"></mspace>
<mspace width="thinmathspace"></mspace>
<mspace width="thickmathspace"></mspace>
<mspace width="negativethinmathspace"></mspace>
<mspace width="2em"></mspace>
<mi>b</mi>
</mstyle>
</mrow>
<annotation encoding="application/x-tex">{\displaystyle a\quad \,\;\!\qquad b}</annotation>
</semantics>
</mathThese values should be replaced by em values. See for example https://github.com/brucemiller/LaTeXML/pull/1962/files
According to https://github.com/w3c/mathml/issues/75#issuecomment-523016332
- veryverythinmathspace => 0.05555555555555555em
- verythinmathspace => 0.1111111111111111em
- thinmathspace => 0.16666666666666666em
- mediummathspace => 0.2222222222222222em
- (2 files) thickmathspace => 0.2777777777777778em
- verythickmathspace => 0.3333333333333333em
- veryverythickmathspace => 0.3888888888888889em