[DiapDealer]

Keep in mind that I'm not suggesting you should comply with everyone's wishes in every aspect. But where your chosen practice does seem to deviate from the Kindle Publishing Guidelines, why not add a warning to that effect?

If you want to personally allow an (x)html cover page, just think about adding a warning that the guidelines recommend against this practice. Then the user (including yourself) can make up their own mind.

Ex: "Your epub includes an xhtml cover page (name.xhtml). The Kindle Publishing Guidelines indicate this shouldn't be done."

Rather than make your personal preferred practices generate nothing but silence for everyone, generate warnings for anything questionable (or errors if the guidelines are specific). What "may" or "may not" work shouldn't really have anything to do with validation. There's plenty of discussions about where there's wiggle-room in the Guidelines. So there's really no need to silently pass "gray areas" in a validation program.

You could also add preference settings that users can use to quell warnings/errors they've chosen to ignore.