Plain HTML and forms for interactivity can be very effective.

In fact, for a long time web forums were largely entirely usable without JS.

See the degradation of GitHub for a great example. You used to be able to interact with most of it without any JS at all; browsing the code repositories, reading and replying to issues, etc. Now it barely shows anything without JS. Of course, I suspect in that case it's deliberate so they can trick you into running a few more tracking and analytics scripts...