To be a little pedantic, it's XHTML. I recently published a book and going through the w3c epub specifications was a pain. Instead, I bought a book I wanted to

To be a little pedantic, it's XHTML. I recently published a book and going through the w3c epub specifications was a pain. Instead, I bought a book I wanted to read then reversed engineered it. For small files you can use the w3c online validator, which will give you an overwhelming list of errors. Note: The kindle does not support epub, instead it uses kpf. For that you have to download a 333 MB program to convert your epubs.