Gregg,
If you are submitting to the Big A, only KDP approval
counts in the end.
AFAIK passing the
real standards does not count with many publishers. You must pass theirs. (I still recommend passing epub check, anyway, but you may need to de-tune your work for a specific platform)
Note also:
Passing anyone's standards, does not insure the work:
looks decent
on the target device
Is free of spelling or word choice errors
does not contain <1,000 of other publishing faults
>
Hitch mentioned a long time ago, her table full of various devices that are used for
Eyeball Mk I checking
That is the ultimate test of YOUR product