Check book is the Validate tool in Calibre. There are others, but this one gets most issues.
BTW I don't think either Calibre or Sigil directly checks the CSS.
Use W3C if you edited or it is unknown. Calibre created CSS always
start out fine
This may be a Kobo thing
RMSDK has no problem with finding nested items.
(there is a slight problem selecting a Parent level file on my old version)
Ch5
Part 2
Ch6
you cant select Part 2 PAGE if you are In the Part 2 nest