Please allow me to cite myself:
Also, you would like to write the formulas directly into the documentation file during editing and also keep them in the compiled files without any separate files — imagine a documentation page with many formulas where the formulas should be rendered fast, compare LaTeX + the KaTeX library which is really fast — BTW faster than the (much bigger) MathJax library. KaTeX is also so small it can easily be embedded into every HTML output (if LaTeX formulas are used).
Note that LaTeX formulas (if they are not too complicated) are also quite readable when you just look at their code, no need to add a separate description, just write them (with appropriate separators) into the documentation source “and you are done”. And in the case of looking at a produced HTML with an according library like KaTeX missing, you still see something that might make sense to you.