Hi Jack, awesome work! Did you write the HTML by hand or did you use or write some static generator? I'd love to have sidenotes like yours automatically generated and positioned.
I can't speak to how he did it, but it looks like he's just using tables and line breaks & divs to separate sections. You could easily do that by hand.
Alternately, you could use Bret Victor's technique from "Magic Ink": write a paragraph followed by a span, typically with absolute positioning. http://worrydream.com/#!/MagicInk
