Nice to see another technologist who laughs in the face of "inevitability", and the relentless assault on the vitality of language (colloquial and technical) by the machine of big-tech/advertising.
Please, stop assuming that everyone's non-mobile viewport is 1920 pixels wide.
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.