"These may have varying features, for example some lacking small caps."
https://en.wikipedia.org/wiki/Baskerville#Digital_versions We use smallcaps heavily as an alternative to bolding everything, but it would be bad if we just use "font-family: Baskerville" and all sorts of unpredictable low-quality fonts out there screw it up. That Baskerville font you paid for probably has them... but how is a
reader's web browser going to use it?
> I do like Tufte-style websites with sidenotes, but these don't show on mobile for this site, despite my tablet having a large screen.
Your iPad Safari reports having a width of ~1180 pixels in landscape mode, even though you have 2x the physical pixels (ie. 2x DPI scaling). This is much too small for sidenotes. Not getting sidenotes is correct. I don't think gwern.net is anything odd in this regard, every other responsive design website you visit must be giving you the same condensed view. Unless you're doing something like disabling scaling? Then in that case I guess maybe it should work and I'd need a screenshot & more information to see what's going wrong. (Maybe a media-query or something.)