It seems that a number of concerns have been raised on this thread. It's not clear that we are ready to ship Animated WebP.
For instance, it seems that Animated WebP is trying to be an improvement over Animated GIF, but based on Alpha's findings, the results are not great. Transport size is marginally reduced at the cost of much greater CPU / power usage. That seems like a questionable trade-off.
The proposal of <img src="foo.webm"> is intriguing. I'm keen on seeing the idea explored further.
At any rate, the recommendation of the Blink API Owners is to work on resolving some of these issues with the goal of building more consensus. (Personally, I'm a big fan of finding some way to improve on Animated GIFs, so I wish you success.)