[irae]

It would still be a good idea to have backend check for this.

- If javascript breaks, (or get blocked by extensions) there should be a check for contradictory answers. - If on future iterations on the UI (more often than backend) logic breaks, backend help improve how to debug the UI update

Just to mention a few reasons.

[iudqnolq]

I'd guess they do. Another commenter mentioned that when you leave feedback or ask for support they log whether you have JavaScript enabled or not.