The website behind this (LEVEL5) is indeed very ancient, but it also has some very good stuff on it. It’s well worth having a look around what they have — I still routinely refer back to their cosmology tutorials whenever I need a refresher.
As I understand it, this was a NASA project in the late 1990s to make astronomical tutorial texts freely available on the web. Clearly a lot of effort was put in at the time: they are often written by authoritative authors and are clear and accessible. However, it looks like funding must have dried up in the early 2000s, and since then it’s become of museum piece.