A UI that's displaying a document ought to be as efficient as a web browser "running" a document, which I think you're saying here uses much less power ("<<") t

A UI that's displaying a document ought to be as efficient as a web browser "running" a document, which I think you're saying here uses much less power ("<<") than a game engine running a simple UI. So it sounds like you're saying that a game engine running a simple UI will drain the battery a lot faster on a laptop or phone than running the same UI in a retained-mode fashion. You may recall that the fact that LÖVE does in fact drain the battery on my laptop a lot faster running simple UIs has been a strong disincentive for me to use simple UIs written in LÖVE!