Before anyone wastes a lot of time like I did trying to use the popover API: it is not ready yet. You can do very basic things in all browsers but positioning is still different and/or totally broken per browser.
Yes, HTML & CSS alone won't replace JS. Of course, for complicated form validation HTML is not sufficient. But IMHO it's very important to provide basic functionality in HTML / CSS as much as possible / reasonable. Moving the functionality to HTML / CSS can potentially improve the SEO.
As for positioning, there is an experimental feature @position-try. Here I made a small demo where it handles overflows.
