Marginalia are marks made at the edges of the page (whether digital or physical). They can be printed by the author/publisher or written by the end user.
In my website I use a specialize
Marginnote React component that is adapted from Tufte CSS.This is what a margin note looks like. This allows me to put marginalia content on the side (or in-line on mobile). This is also how I choose to handle footnotes, although in a book footnotes would of course go in the footer of the document.And this is a footnote.
I got the idea from Maggie Appleton’s website.