Why can't we just use MP4? Add an inline JavaScript decoder for Mobile Safari, and then at least all the other browsers get instant hardware decoding. Are there trust issues with allowing third-party MP4 embeds into user-submitted content such as forums/hosted blogs? Do we need a "no sound"/"max file size" attribute on the video tag?
Hence why there is a push for alternatives to Gif/APNG and mp4/webm. We want a video-like animation, that plays on page load.
Now, the real question is, why don't mp4s play when we hit play?
I can only speculate, but I think it's because mp4s are streamable media, where a troll can simply decide to keep streaming mp4s and drain the user's bandwidth. (Never mind the fact that it is also a problem with Gifs.)