> That's because paper used to print books isn't always white.

Isn't it basically rule #1 of UX design to never use pure white / pure black?

Yes, that's what's being discussed here. I'm disagreeing with the people who are defending using pure white as a background in light mode.

Pure black is more understandable, because it helps with battery life on mobile and notebooks, although I believe it shouldn't be the default dark mode.