Enable "MathML (experimental; no images)" in Special:Preferences → Apperance → Math. Then preview a page with the contents <math>a\not=b</math>.

This renders as a=b but it should render as a≠b. This is the MathML generated:

<mstyle displaystyle="true" scriptlevel="0">
  <mi>a</mi>
  <mo stretchy="false">=</mo>
  <mi>b</mi>
</mstyle>

Note it uses = instead of ≠.

For comparison, https://temml.org/ generates

<mrow>
  <mi>a</mi>
  <mo>≠</mo>
  <mi>b</mi>
</mrow>