We have a post in the News section with some detail about our tech stack, which is all based on free/free tier services. One of the big goals is to make something reproducible in a way that provides value to readers and authors alike. That's not an easy task in the world of academic/educational publishing!
Any comments welcome. HN folks may also enjoy the Rhythm and Expectation article, which includes some interactives using ToneJS. Maybe that's worth a different post.
Show HN: World Music Textbook, a collaborative open educational resource
worldmusictextbook.org