There's a maximum width that humans are comfortable with, reading paragraphs of text - usually measured in characters rather than inches though. Designers disag

There's a maximum width that humans are comfortable with, reading paragraphs of text - usually measured in characters rather than inches though. Designers disagree on the precise limit, and how to count "characters" in variable-width fonts, but "don't go over 80" is a good start as a rule of thumb. That is why newspapers print in multiple columns, novels are printed on smaller page sizes than the usual Letter/A4 (depending on continent) and why the default margins for TeX' article and related classes are so large compared to your average word document. It's also why variations on `main {max-width: 800px; margin: 0 auto;}` are so common on the web (along with all kinds of multi-column layouts). If you're on a medium that's wider than this limit, then sidenotes are absolutely a good use of space - like a 1000+ pixel wide browser window, or physical textbooks (Crafting Interpreters, whose author gwern also mentions for their previous book on game design, is an especially good example of this). If your medium is a mobile device, then you don't have that much "side" to put the notes in, so you have to get creative another way (you also don't have the same "hover" semantics as on desktop). (End note: personal hobby horse of mine, but I hate "let's increase the width of our code from 80 to 120ish as everyone has widescreens now". Not only do I find a single file harder to read that way, but the main benefit of a wide screen for me is that I can have two panels open side by side at the time.)