Here is the picture of the effect.
I would pay for source of all the weird site stuff Gwern has commissioned / developed over the years. I am never sure how much of it it’s worth it to emulate in my own site; for instance, cool sidenotes of the kind he and tufte-css have wouldn’t fall back as neatly for text-only browsers as the stuff I have now. (counterpoint: I am pretty sure I’m the only person I know to have visited my site with a text-only browser)
Probably better to focus on my content, but by gosh it’s cool to see people making this stuff happen.