From @ways2read:

From my interpretation of the 3.3 spec, specifically at https://w3c.github.io/epub-specs/epub33/core/#sec-cover-image it should be possible to use a webp image for the cover.

In my manifest I have

   <item id="cover_webp" href="media/cover.webp" media-type="image/webp"  properties="cover-image"/>

But EPUBCheck 5.0.0-beta-2 reports:

Validating using EPUB version 3.3 rules.
ERROR(OPF-012): test.epub/EPUB/content.opf(-1,-1): Item property "cover-image" is not defined for media type "image/webp".

Check finished with errors
Messages: 0 fatals / 1 error / 0 warnings / 0 infos

Looks like a bug with EPUBCheck.