One example: in my static blog I provide very nice maths using MathJax, but I also provide fallback PNG renders of the formulae. The small JS my blog has, it reads these pictures' alt texts and renders the latex if found. This stuff is not rocket science, people just don't want to spend time on this kind of stuff.

You might want to switch to katex instead - and katex can also be run on the server to return HTML directly.

KaTeX looks very good, but currently I let org-mode do the backend lifting for me. Maybe if I rewrite my blog engine once again... ;-)

At least use it on the client then – it’s a lot faster than MathJax :)

Clearly that's an exception, I don't think that really needs to be discussed or considered when talking about static blogs not needing JavaScript.

That would be a reasonable exception. Of course, I'd only give the page a 5/5 rating if the JS code would be still readable even if no output would be produced.

Echoing others, as blog posts go, that's the exception and not the rule. Inline demos are furthermore not "primary content" but supplementary material.