There's quite a lot you can do with just CSS using hidden inputs for state and clever selectors to dynamically update content. It's possible to create a non-JS

There's quite a lot you can do with just CSS using hidden inputs for state and clever selectors to dynamically update content. It's possible to create a non-JS tab implementation using just hidden inputs and CSS for example, although I wouldn't recommend it. 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.