> Alternate stylesheets don't persist (at least in Firefox), which is really annoying.

Yes, I also think they should. What I wondered, is this behaviour prescribed somewhere? Even if it were, form content is retained even across randomly typing in a different URL and navigating back, form submission and even across real crashes just seconds after I typed the letter. This is really nice and I don't think it is mandated. This is the stuff User Agents should be compete with, not ways to get data from the User to a surveillance company.

> Being able to provide a night mode alternative without even asking the user, assuming they've set the OS to their preference, is really seamless.

Yes, that's nice and how every thing in a UI should be. It helps both the programmer and the user in efficient and controllable usage.