Aaron T. Grogg has a nice page chock full of examples of UI, which used to be the sort of thing that we’d use JavaScript for, but can now be done in HTML & CSS. No hate:
I have nothing against JS, but it has better things to do
The examples are very modern, like Tabs, where the actual tabs are created with a ::scroll-marker and positioned as a ::scroll-marker-group with anchor positioning. Fancy.
