My two cents: Which version of webp should Firefox support? There are 3 major versions, and the subtleties in the 'extended' webp is only traceable through the releases of Chrome.

further, how can I tell which client supports which version of webp? `Accept: image/webp` is insufficient because it doesn't tell me which version or subset of features this client uses. Take for example, ios9b1 where webp was thought to have accidentally been available through CoreImage. It only supported WebP basic. But how would a server know this?

Lastly, webp is not setup to be successful for the next set of requirements: it is 8bit per channel color only (can't do 10bit needed for P3 or rec2020). It can't do full chroma subsampling (4:4:4). And the list goes on. It was a good first start, but it needs much more work before it can break out of the Google grip and get wide support.