He wrote "can be generated". You can use KaTeX for setting up complete static web pages with no JS/server-side prozessing at all. Example from my blog:

https://depot.traits.de/pages/2020/02/21-markdown-example.ht...

For the vast majority of use cases, you should probably be pre-generating these anyways, and for the few use cases where content would be constantly changing, client-side rendering makes way more sense.

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.