I try that in uBlock from time to time, but I still can't seem to get it quite right. Testing in Duolingo right now, I set it to get rid of the app popup, as well as the semitransparent overlay that darkens the whole page. But there's still some element on the page that restricts me from scrolling until I tap on the page. It's an improvement, but still not quite there yet.
I had to look up 'html modal', yeah it sounds like the same thing. I learned web dev back in the xhtml days. Back then those kinds of boxes were only beginning to see popular usage, and there was no official tag for making them.