This is extremely fun. Jeff Lindsay has created Topframe, and writes:
The default screen after installing has some hardcore weirdweb energy.
Mac-only I think for now. But it’s super easy to customize. It’s just an index.html file at ~/.topframe/index.html. It auto-refreshes when you save the file.
The other hiccup I had is that it messes with taking window screenshots. You know how you can go Command + Shift + 4 then Space on macOS to select just one window to screenshot? You can’t do that with this running because it’s always the top window. Hence “top frame” (get it?). I think it would be more fun to be “bottom frame” in that it would act more like a background instead of being always on top.
Apparently, Windows could do this a long time ago, proving (yet again) that there is nothing new under the sun.