Digital Marginalia
The print tradition of marginalia—notes written in the margins of a book—translates powerfully to the wide screens of the modern web.
Unlike traditional footnotes which require the eye to travel to the bottom of the page (or click away to another view), marginalia places context adjacent to the triggering text.
This pattern requires careful responsive design. On narrow screens, marginal notes must collapse into inline expansions or traditional endnotes.
Sidenote: The Tufte CSS framework is a famous implementation of digital marginalia, heavily inspired by Edward Tufte's printed books.