From what I've seen you can use only html, you can probably put css and js in it but it's not the best way to make a website or web app.

It from the source code of nostrsite It also seem to manipulate the html to inject code from CDN, and they are behind a pro plan, not a bad thing per se, but you're limited with what you're allowed to use.

What is bad though is that it uses kind: 1 for the code. It'll spam the feed on normal clients with HTML note on the relay.