> Login just to do this would definitely not be a good idea, for all the
> reasons you mention.
>
> Is it possible to implement everything client-side, using LocalStorage?
> This will not be perfect privacy-wise either, since there might be
> resources that are loaded differently depending on settings, but "fully
> client-side settings" could be an acceptable exception.

Could we have a UI convention for buttons that save a simple preference
to local storage? I mean that those buttons would look approximately the
same across different scheme.org subdomains, and would have similar text.

So users would come to expect that if they click on a button like that,
a simple preference will be saved even without login?