http://9gag.com/gif used to show the animations as gifs, now they are html5 videos. What is the reasoning behind such a decision?
The reason is simply that video compresses better than gif in many cases, particular when the gif is of some size or length.
Additionally, video can be streamed affecting traffic and when the displaying can start (almost right away), gifs has to be loaded completely before they can be shown (ore they will be shown slowly and progressive).
Now that most browsers are able to show video natively, video becomes a viable and desired option to animated gifs.