While I agree in terms of modern browser expectations (and books absolutely should not need JavaScript), I think books in HTML makes a lot of sense. HTML was meant for sharing text documents, after all.

epub is overkill for a vast majority of books.

A format that only supported

- headings

- paragraphs

- emphasis (bold/italics)

- bullets

- inline images

is good enough. A simple container with a TOC pointing to text blocks/files within it that can be processed very cheaply.

Unfortunately, with something like epub, you lose all the simplicity because you want to support every single feature even if rarely used.

HTML and CSS is enough for 99% percent of epubs, and that's the only subset of epub standards that Crosspoint tries to support