The “summary” element for showing/hiding details springs to mind. What else is there?
[0]: https://www.digitalocean.com/community/tutorials/css-collaps...
You can also do basic client-side form validation with just HTML (although you'll need some CSS to display the errors I think).
The summary element is a good example and the first that came to my mind. Most of the things I can think of would require at least some CSS, or are already reasonably well known.