I think @kovidgoyal 's reply hits it well, thank you!
From the link:
Quote:
The EPUB I produced with calibre is not valid?
calibre does not guarantee that an EPUB produced by it is valid. The only guarantee it makes is that if you feed it valid XHTML 1.1 + CSS 2.1 it will output a valid EPUB. calibre tries hard to ensure that EPUBs it produces actually work as intended on a wide variety of devices, a goal that is incompatible with producing valid EPUBs, and one that is far more important to the vast majority of its users. If you need a tool that always produces valid EPUBs, calibre is not for you. This means, that if you want to send a calibre produced EPUB to an online store that uses an EPUB validity checker, you have to make sure that the EPUB is valid yourself, calibre will not do it for you – in other words you must feed calibre valid XHTML + CSS as the input documents.
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