[kixiQu]

I think this doesn't work well even if you have a really large screen. It's doing a style somewhat like https://edwardtufte.github.io/tufte-css/ but doesn't scale the content to be sure it can fit on the screen, nor center it without having to scroll for sidenotes.

Just in case the author sees: with the way the sidenotes are specified relative to the parent, this is [probably] better:

@media (min-width:760px) { #layout { max-width:min(80ch, 55vw); margin-left: 4%; } }

(you can fuss with another clamping function for the margin-left to make it center properly on Very Large screens if that's a concern, but, uh, I don't want to bother)

[probably]: I mean, it might be an expressive choice to make the sidenotes something you have to page back and forth for, but then typically footnotes would be simpler, so I'm guessing this wasn't intentional.