It's not only a great reference documentation, it also has comprehensive guides for both beginners and advanced developers. Including niche/complex features like canvas, webrtc, service workers etc.

In a web programming course I created I often referred to and leaned on MDN.

Aside:

DDG lets you search in MDN directly if you prefix a search with !mdn. It's one of the few sites that has a very much usable search feature.