Printing books directly from the browser isn't a solved problem, so it seems like a good project, to me.

I don't see any evidence on their website this is intended for reading in the browser, and tons of clues that it's intended for printing books from the browser. I mean, there are tools for physical page sizes, blank pages, front/back matter, page counters, etc. Things that make zero sense in the browser and are necessary for a nice book printing.

So, what makes you think this is intended for reading in web browsers? As you note, it's horribly unpleasant in a browser...seems like they would have noticed that if they were testing in the browser, rather than using it to print books.