Remember; there are two distinct renderers on Kobo devices; one for standard ePubs, the other for Kobo format ePubs. The two engines are based on different code bases and have different defaults. ePub support is based on Adobe's RMSDK; Kobo ePub on NetFront's ACCESS engine.
I believe that any of the PC Kobo applications ONLY use ACCESS.
Have you looked at
https://github.com/kobolabs/epub-spec ?