Enable "Client side MathJax rendering" in [[https://www.mediawiki.org/wiki/Special:Preferences#mw-prefsection-rendering|Special:Preferences → Apperance → Math]]. Then preview a page with the contents `<math>\operatorname{arg\,max}</math>`. This should render similar to `<math>\operatorname{argmax}</math>` but with an extra space between "arg" and "max". Unfortunately, unwanted space is inserted between every letter, and it looks like "a r g m a x" instead of "arg max".
This is because in the generated MathML every letter is put inside its own `<mi>` element:
```
<math xmlns="http://www.w3.org/1998/Math/MathML" class="mwe-math-element">
<mrow data-mjx-texclass="ORD">
<mstyle displaystyle="true" scriptlevel="0">
<mrow data-mjx-texclass="ORD">
<mi data-mjx-texclass="OP" mathvariant="normal">a</mi>
<mi data-mjx-texclass="OP" mathvariant="normal">r</mi>
<mi data-mjx-texclass="OP" mathvariant="normal">g</mi>
<mspace width="0.167em"></mspace>
<mi data-mjx-texclass="OP" mathvariant="normal">m</mi>
<mi data-mjx-texclass="OP" mathvariant="normal">a</mi>
<mi data-mjx-texclass="OP" mathvariant="normal">x</mi>
</mrow>
</mstyle>
</mrow>
</math>
```
This does not happen with `<math>\operatorname{argmax}</math>`:
```
<math xmlns="http://www.w3.org/1998/Math/MathML" class="mwe-math-element">
<mrow data-mjx-texclass="ORD">
<mstyle displaystyle="true" scriptlevel="0">
<mi mathvariant="normal">argmax</mi>
</mstyle>
</mrow>
</math>
```
I should stress that both look good when rendered with Firefox or Chromium. So this could actually be** a bug in MathJax.**