If you are serious about e-books, then XHTML/CSS is a must. Almost all e-book formats are more or less based on it.
Perhaps the fault is in Calibre, but not necessarily. Could be the input. In any case, if you open it in Sigil and save it perhaps the error is solved. Sigil is designed to save only structurally correct ePUB. It might correct your error automatically.
Be aware, content based errors will not be solved automatically but you can use the internal validation for that. The internal validation is the same as the standalone FlightCrew.
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