For Google Chrome.
Chrome users should always download extensions to get these benefits. 
I've been using this add-on for some time but you've been ahead of me posting it.
Animated avatars aren't supposed to be allowed, but I never got around to adding detection for the APNG ones. I'll look into it.
APNG have a "acTL" chunk in them. So, if you search for the string acTL (or, in hex, 61 63 54 4C (the 4 bytes before the chunk marker (i.e. 00 00 00 08) are the size of the chunk in big endian format, without counting the size, marker, or CRC32 at the end of the field)) you should be pretty good. To get it even better, check that this chunk appears before the first occurrence of the "IDAT" chunk (just look for IDAT).
http://foone.org/apng/identify_apng.php