Looking into a plug-in internals, I just found
IMAGE_FILES = ['.png','.jpg','.jpeg', '.gif', '.bmp', '.svg']
NO '.webp'.
Moreover, trying to update a book cover in calibre with a .webp file, it got silently converted to .jpg
Now, I have collated an epub in sigil (a guide for some Gimp plug-ins) from html pages, and both the cover and a part of the illustrations--particularly, all the animations--are .webp: it works flawlessly in both calibre e-viewer and Book Browser.
To get Sigil to deal properly with the images, I was asked to supply the MIME type, which per wikipedia is image/webp.
I dare say not all devices will accept the format--but that shouldn't put a restriction on everybody...
And, yes: it is not included in the epub 2 specification... and so isn't much CSS3, even when it works for many of us.
