EPUB
An e-book as a website in an archive.
Where it came from
EPUB emerged in 2007 from the Open eBook Publication Structure effort by IDPF; since 2017 the standard is maintained by W3C (after merging with IDPF). Version 3 dropped legacy XML in favor of plain HTML5.
It's the most widespread e-book format — everything understands it except Kindle (Amazon keeps pushing its own formats).
How it works
An EPUB is a ZIP with mandatory META-INF/container.xml pointing to the OPF manifest: chapter list (XHTML), metadata, reading order, CSS. Readers render chapters as web pages with reflowable layout — changing font size re-flows text instead of zooming.
Consequence: converting EPUB is essentially working with HTML. UWC uses exactly that — chapters are unpacked and converted like ordinary HTML.