Kaoru AoiCard art can use animated media for greater effect.
mp4, gif, and apngs are supported.
The current recommendation is to use optimized and compressed apngs. It is the best format for when considering both quality and performance.
Multiple mp4s on a single page hurt rendering performance on low end systems. We may see this improve over time as browser makers fix their rendering. gifs are generally performant, but image quality is low, however it can be a good solution for simple cartoon images without too many colors.
apngs have neither problem. Before uploading, try compressing them through a service such as https://tinypng.com/