Meta-comment: I really love the expanding explanation hyperlinks in this article. It's a much better alternative (imo) than say, linking in a new tab, or a pop-up, etc.
I think it might be even more useful with a faint background color so you could skip ahead to the end of the parenthetical if you figured out what you wanted to a few words in. Overall really cool feature though, which I'm tempted to steal haha.
A side note: I still plan to read your Geometry for Programmers, now that it is close to being finished. All the best!
But not on this page. I thought that the arrows are a bit noisy. I can still add this "toggle back" feature here if you want.