Enable "MathML (experimental; no images)" in Special:Preferences → Apperance → Math. Then preview a page with the contents <math>x := 1</math>.

This renders like x: = 1 in Firefox (see figure 1) but it should render like x := 1 because the := operator is used to define the variable x. In client-side MathJax mode it looks good.

This is the MathML generated:

<mi>x</mi><mo stretchy="false">:</mo><mo stretchy="false">=</mo><mn>1</mn>

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

<mi>x</mi><mo lspace="0.2222em" rspace="0em">:</mo><mo lspace="0em">=</mo><mn>1</mn>

and https://latexml.mathweb.org/editor generates

<mi>x</mi><mo>:=</mo><mn>1</mn>