Yeah there's a technically unofficial specification for animated PNGs, it's actually a bit old. It's nothing new.
Despite it not being official, I think all major browsers support animated PNG and it is the best widely-supported animated format available. Right now browsers support GIF, APNG, and animated WEBP but GIF is absolute trash (shit compression and limited palette) and WEBP is absolute trash in a different way (no support for frame combining so filesize is crap). However, if you wanna see what an actual good format looks like, check out FLIF. Their lossy, lossless, and animated formats are better than absolutely any image codec out there right now. Unfortunately, nothing supports FLIF though.
Sorry for rambling, I like codecs. Well, good codecs that is.