Okay, so it's not LaTeX, just math typesetting that looks like LaTeX and takes LaTeX input.

I'm more interested in solutions that work with the broader LaTeX ecosystem (like SiUnitX or amsmath).

Mathjax does support a whole bunch of common latex math related packages [1]. SiUnitX is notably missing from the inbuilt extensions, but a port to version 2 is available [2]. Should be updated to version 3.

[1] https://docs.mathjax.org/en/latest/input/tex/extensions/inde...

[2] https://github.com/mathjax/MathJax-third-party-extensions

Thank you, I wasn't aware of these extensions.

Ahh. I was confused for a moment why \def wasn't working.