This is great! We need more alternatives to the full calibre stack for working with eBooks. Are you planning to keep it epub focused or would you consider adding support for something like azw3? Also is there a conformance suite for EPUB?
I am happy to extend the functionality, can you maybe create an issue on github.
I am mostly concerned with how big that functionality will end up being, but if its not too much, ill happily add it or at least leave the issue up for someone to pick up.
In terms of conformance, using https://www.w3.org/publishing/epubcheck/