Here is an interesting document to read about all of the limitations of kobo kepub and epub versus the actual epub spec. Kobo is quite a limiting format with no css page break support on their desktop ereaders and lots of issues, restrictive file naming conventions, against all use of em for margins, against use of javascript for formatting documents, etc, etc.
Kobo really needs to update its e-reading software to something using real webkit engines.
https://github.com/kobolabs/epub-spe...ster/README.md
As it stands, I think kepub would best be handled by input and output plugins in Sigil as they have real limitations and do not use epub3 landmarks/semantics or even use most of the metadata a normal epub3 would provide.
Not a format I would choose over standard epub3 just based on its long list of limitations alone.
Certainly, on the Desktop, Thorium would be a better e-reader once stripped of kobo drm.