Thanks for all the explanations. I always thought it was regular HTML, but now I know to watch out for the differences.
Can you say a few more words about the library https://github.com/standardebooks/tools ? Can it generate ePub3 from markdown files or do I have to feed it HTML already. Any repo with usage examples of the `--white-label` option would be nice.
The tooling does two main things: create a valid epub3 skeleton for your content, and build your book into “compatible”, Kobo and Kindle versions. You need to supply the valid XHTML.
There’s more info on the build process at https://news.ycombinator.com/item?id=46469341