I haven't looked into it for several years, but animated PNG (APNG) looked really interesting. I have no idea if support for the format was finally added to browsers without needing an extension or not. I need to look into it again sometime soon. Related to it: https://web.archive.org/web/20230717013711/http://littlesvr.ca/apng/ https://web.archive.org/web/20230717013713/https://apngasm.sourceforge.net/