WebM (despite being closely related to WebP) can be 5-10x more efficient than WebP.

That's because WebP focused so strongly on being a GIF equivalent that it also dropped all things that made WebM efficient and instead adopted GIF's awfully inefficient architecture (just dumb frames overlaid on top of each other, without motion vectors or predicted frames).

Safari shows how it can be done: it supports silent MP4/H.264 straight in <img>. You get all the ease of use GIF, order of magnitude smaller file, and hardware acceleration.

It's unintuitive, but well-compressed videos are cheaper to decode than dumb "animation" formats, because file size differences are so massive that it's cheaper to decompress a small amount of complex data than to chew through vast amounts of poorly compressed data.