Theme switchers! My favorite example of why "document-only" sites might need JS. Sure, you can use @media prefers-color-scheme for defaults, and you can do checkbox/radio shenanigans to make it switchable, but there's no way to persist that preference on a static site without JS.

If you don't care about your site being static then you can track it server side, but that has big implications for his you can just your site. Sticking a little flag in localStorage really is the best solution, if you can get past ideological purity.

Most sites also don't need forms at all either, at least for normal users.