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