Ah. I didn't even check that, and it's the first I've heard of such a beast, too. And checking on that, yeah, APNG is a thing, though it's not sanctioned by the PNG standards people and support is spotty. So if you looked with Internet Explorer, Safari, or Chrome (oddly) without the right plug-in, I apologize for the confusion.
(I assumed it was typical dynamic web stuff, like Google Maps. Go figure. Or maybe it did work for you, since the Wikipedia page says that there's a standard library to "fake" an APNG for browsers that don't support it.)
Oh! That explains the animated site icons I've seen...