Just what you wanted to hear, I'm sure!
In the page on the real projective plane, paragraph four, ... throw away the lower hemisphere, not through.
BTW, I'm finding your article to be fascinating both mathematically (there's still a lot of fuzz, but I'm working on it) and instructionally. If all mathematics were written in hypertext more students might actually read it. Where clarification is needed the term is right there to be defined. This is exciting!
This is the first hypertext document I've read, but think you've done a great job!