“Tufte-CSS: Sidenotes: Footnotes and Marginal Notes”, (; similar):
One of the most distinctive features of Tufte’s style is his extensive use of sidenotes.3 Sidenotes are like footnotes, except they don’t force the reader to jump their eye to the bottom of the page, but instead display off to the side in the margin. Perhaps you have noticed their use in this document already. You are very astute.
Sidenotes are a great example of the web not being like print. On sufficiently large viewports, Tufte CSS uses the margin for sidenotes, margin notes, and small figures. On smaller viewports, elements that would go in the margin are hidden until the user toggles them into view. The goal is to present related but not necessary information such as asides or citations as close as possible to the text that references them. At the same time, this secondary information should stay out of the way of the eye, not interfering with the progression of ideas in the main text.
…If you want a sidenote without footnote-style numberings, then you want a margin note. Notice there isn’t a number preceding the note. On large screens, a margin note is just a sidenote that omits the reference number. This lessens the distracting effect taking away from the flow of the main text, but can increase the cognitive load of matching a margin note to its referent text.