Without having looked at the text in depth, I applaud the idea here - I'm convinced the web browser can help us improve upon maths textbooks printed on paper, but there aren't many 'hypertextbooks' like this in existence (partly, I think, because most people positioned to author a maths textbook want to use latex; and most people who are able to develop a web app aren't positioned to author a maths textbook).
I'm interested to know how the authors funded this, and if they have any data about the impact it has had.
I'm interested to know how the authors funded this, and if they have any data about the impact it has had.