I want semantic HTML sidenotes

I like my blog posts with sidenotes. By sidenote, I am thinking of text that’s not independent enough from the main text to rate as an aside. For example: clarifications of words that I think most of my readers will know or personal stuff that I want to add, but keep from the main story. They're like footnotes, except that on the web, that's a bit of a typographic archaism.

I put the sidenotes in the margin of my text column. On small windows, I hide them. An underlined, clickable, link-like span allows the user to make them appear. I have a workaround, but it's not 100% accessible and requires funky HTML, CSS and JS to work.