Ah, that seems exactly like I was asking for, unfortunately in the 3 years since Safari added it, I can't find much pick up on it. I found this on Chromium but with not much progress. It seems their code path doesn't quite let img decode a video, I assumed it would be as easy as creating a fake <video muted loop autoplay> but it's probably much more messy.

https://bugs.chromium.org/p/chromium/issues/detail?id=791658