"We need an image file format that is, in essence, a storage locker." I disagree. HTTP supports cache-able compression, and connection re-use. It wouldn't be

"We need an image file format that is, in essence, a storage locker." I disagree. HTTP supports cache-able compression, and connection re-use. It wouldn't be any slower to download frames as needed (lazy = good)), using the existing transport protocol. All you need is an ASCII/UTF-8 index file for an animation. Getting browsers to support anything new that doesn't come from the W3C and contain the word "semantic" in the spec on the other hand... That's the hard part.