I'm confused what aspect of this you're mad about. MNG is an animated format? Well, Firefox has supported APNG (Animated PNG) for the past 17 years without it ever being standardized and it has become extremely widely adopted. And... this new spec attempts to standardize it.

Removed MNG and started work on APNG 20 years ago! https://bugzilla.mozilla.org/show_bug.cgi?id=257197