The World Wide Web Consortium
recommends using dedicated bidi markup to describe RTL content, where markup is available and epub books with RTL styles will fail EPUBCheck.
For example,
Code:
body {direction: rtl;}
will trigger the following EPUBCheck error message:
Code:
ERROR(CSS-001): The 'direction' property must not be included in an EPUB Style Sheet.
@GarbageGhost
Try replacing all <body> tags with <body dir="rtl"> tags.