The thing specifically with KaTeX is that if you do this, you can blow page sizes out completely. In the example above, less than 20 characters of latex turned into a giant amount of HTML markup. Repeat this in many places for a mathematics-heavy page, and the browser might be better off just running KaTeX itself. Besides, you need to ship a whole bunch of fonts and a stylesheet anyway.

KaTeX has an 'output' option and one of his settings is 'mathml'. You can produce clean MathML this way and even mixed HTML/MathML output (default and what you see in the OP's example) with MathML for accessibility. Problem with the pure approach isn't KaTeX but the browser support.

Oh yeah, definitely, I was just responding to the "calling server-side JS" part. Even if the same library was implemented in native code, you still wouldn't want to be calling it inline from a cgi* script.