[asimpletune]

<video> and mp4 can also work well. As a GIF replacement, the initial differences are minor because you can easily add loop and autoplay to the video’s attributes, and both work with most browsers’ reader modes. From my tests, though, video-as-GIFs isn't a feature supported by the major RSS readers, while WebP works without any issues.

Anyways, the real insurmountable problem is animations with transparent backgrounds. There’s no good way to do this in mp4, and WebP seems to be the only format that does it well. But because of all the little pains, headaches, and surprises that can arise due to subtly different use cases, I still think WebP is the format that will give you the most benefit with the least drawbacks for silent animated images. Even without having transparent backgrounds as a requirement.

[Flimm]

MP4 is just a container format (like .tar or .zip in some ways). The video codec is more important information than the video/audio/subtitle container.