Are you still working on epub export, @matt ?
I found some bugs, that might benefit from your attention. 
In order to reproduce them, download https://write.as/blog/tag:Updates.epub and validate with EPUBCheck.
On my machine it resulted in 41 error messages.
Duplicate elements:
- 3x
RSC-016 Fatal Error while parsing file: Attribute "id" was already specified for element "h2".
Forbidden elements:
- 1x
CSS-001 The "unicode-bidi" property must not be included in an EPUB Style Sheet
Missing elements:
- 1x
OPF-053 Date value "" does not follow recommended syntax as per http://www.w3.org/TR/NOTE-datetime:zero-length string. - 1x
RSC-005 Error while parsing file: character content of element "dc:date" invalid; must be a string with length at least 1 (actual length was 0) - 1x
RSC-005 Error while parsing file: Element "title" must not be empty.
Remote resources not properly referenced or declared:
- 5x
RSC-006 Remote resource reference is not allowed in this context; resource […] must be located in the EPUB container. - 4x
OPF-014 The property "remote-resources" should be declared in the OPF file.
Broken links:
- 18x
RSC-007 Referenced resource […] could not be found in the EPUB. - 7x
RSC-026 URL […] leaks outside the container (it is not a valid-relative-ocf-URL-with-fragment string)
From my own experiments with pandoc I’ve learned that creating a properly formatted epub has its unique challenges. It might sound trivial, but an epub is not a website. It is a self-contained entity with a peculiar structure and wrapping.
Hope this helps.