The same it is for everyone since the beginning of WWW: a static page with images. The web is uniquely unsuitable for anything else.
So forget e-commerce? Amazon didn't do too badly and CGI was hot on the heels of HTML so I think "everyone" is a little exaggerated.
> So forget e-commerce?
Says (s)he and immediately follows up with
> Amazon didn't do too badly
My point is that the WWW was a lot more than static HTML and CSS pretty soon after its inception.
You're confusing displaying different information at different URLs with being "more than static HTML and CSS".
This is a very old fashioned point of view I’m afraid, things have moved on.
May I ask how to create a Mathematics Stack Exchange clone without JS?
PHP and cocaine.
What part of a Mathematics Stack Exchange clone would require JS?
In "modern" stacks it is preferable to do things like layouting of formulas millions of times on the clients, instead of once on the server. I guess that sort of thing needs JS.
As you have pointed out, but formulated more explicitly: client-side rendering should deal only with HTML/CSS because that's what the browser is built and optimized for. Every line of script changing the DOM (html structure) may trigger a redraw of the page, which means wasting considerable amount of resources! But even if your script outputs HTML only once, you still have O(n) HTML templating instead of O(1) for n clients. Such a waste!
The MathJax part.
MathJax is somewhat convenient but doesn't seem necessary at all.
In the past Latex symbols were rendered on server as an image.
It was not pretty but it works virtually anywhere.