The idea seems to be inherently "UI Composition in pure HTML", at least from the user's perspective. Which, sure, it's possible. But that's not a great idea. And the video is just how HTML works already. Firefox has had a neat inspector tool that showed you all that for years. "A div that loads above the rest and is anchored to the window" isn't new.

The concept behind layerJS is very fundamental, so fundamental that one even thinks its already there. It must be there, but it's not. We looked deep. So we created a polyfill (please see my previous response). Of course you can code every effect that layerJS can do manually. But then you would need to do so...