It’s not. It is up to the owner to code it into their website or not.

ARM chips aren’t better at power efficiency compared to x84 above 10 or 15W or so.

Do you have a source for that? It seems a bit hard to believe.

But isn’t there x86 emulation for those edge cases?

If you didn’t want to game on your laptop, would an ARM device not be better for office work? Considering they’re quiet and their battery lasts forever.

This assumes everyone has the same destination, which is rarely the case