Excellent! I had this same idea in 2011, kudos on the execution! The size difference between animated GIF and actual video codecs is vast and the idea seemed obvious. I made a quick mashup with ffmpeg+Django (I'd use Go/MongoDB for web instead of Django these days.)

HTML5 video wasn't uniformly supported across browsers (and remember Google announced Chrome would drop h.264 around that time) and I didn't get positive responses on the idea. Maybe with more tools to support HTML5 video in animated GIF use cases and educational tools showing the difference in loading times and encoding algorithms this idea could take hold.

Also the WebP engineers have developed an animated WebP format which I disagree with but could be a step toward breaking GIF's monopoly on short animations.