I think a better idea would just to allow embedding theora, webm or mp4 video clips as img sources. While APNGs are pretty cool in that they're lossless and support transparency, it still suffers from the same fundamental problem as animated gifs which is that there's no interframe compression, leading to insanely large files for anything but the shortest throbber icon loop (which should probably better be done with css transitions anyway).
HTML already has some tag weirdness with the fact audio is technically equivalent to video, making img join the party wouldn't be bad and it'll certainly save lots of bandwidth and improve visual quality for those memes.