I suspect that this was caused by confusion over what was meant by should.
From the epub 3.2 specification section 2.2 Content Conformance:
Quote:
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The XHTML Content Document filename should use the file extension .xhtml.
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and from RFC2119:
Quote:
3. SHOULD This word, or the adjective "RECOMMENDED", mean that there
may exist valid reasons in particular circumstances to ignore a
particular item, but the full implications must be understood and
carefully weighed before choosing a different course.
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This file properties recommendation was removed in the epub 3.3 specification.
BTW, the various generations of the epub specifications are hardily recommended as a cure for insomnia.