This doesn't look like it gives websites something they couldn't do before with CSS and JS.

I'm pretty sure it does but maybe I'm wrong, I had never tried to create popovers/popups using CSS. What's neat[†] about the popovers API is that it doesn't need any JS, you just write a couple of HTML attributes and voila, a popover/popup. They will run with JS disabled as well.

https://chrisburnell.com/html-can-do-that/

[†] Neat for me building things like tooltips and dropdowns into my web apps without using JS; not neat for annoying people with popups.

The core trick to making a popover, tooltip, or, really, any kind of overlay element that's anchored to something on the page is putting an element with `position: absolute` inside one with `position: relative`. You can make it display on hover with CSS alone too, if you really want to.