Having seen animated gifs, I can assure you that mngs (pronounced as mings) would be likewise huge. It's better to just have a 1-line JS script do this, and a standard for sandboxing nonthreatening JS.
Animated GIFs are getting very popular again because even with 5Mb-8Mb sizes GIFs are easier to handle today, comparing to downloading that 200Kb Dancing_Baby.gif in the 90s.