I can see the use, but this is also kind of wild. It's live-updating if you select text, it's almost literally a separate viewport into the same content.

I'm trying to figure out the use-cases and implications of this. You can do offscreen/overflow:hidden tricks to make it work with hidden elements. But... as far as I know there's still no way without a library to convert that element to an image (for privacy reasons I suspect).

I'm sorry for kind of being all over the place, but the MDN page is pretty short and I have a lot more questions. I guess I could always apply CSS filters and distortion directly to elements, does this being treated like a background open up possibilities there? It's the first time I'm hearing about this even though it's been around for a while, and I wouldn't have suspected this would be a CSS feature even on the table. How useful is this beyond just cloning viewports (assuming I'm only targeting Firefox and don't care about Chrome)?