> why will we be supporting both WebM and animated WebP?
From: https://developers.google.com/speed/webp/faq (Why not simply support WebM in <img>?)
WebM uses more memory, it is less widely supported, uses more resources when playing, and is currently less efficient compression-wise than WebP.
About FLIF, I was recently checking it and it's indeed a very interesting format. Notably because it can be used on any type of image. I assume it hasn't pickep up steam because it doesn't have the backing of any big sponsor. Google is pushing WebP, and introducing another format at this time is probably not worth it.