APNG is widely supported, add to spec, register image/apng

[svgeesus]

Animated PNG, a GIF-like animation extension for PNG, originally failed PNG-Group standardization. There were two reasons: firstly because some preferred to continue to promote MNG, a complex but fully-featured alternative animation format. Secondly because the original proposal suggested sharing the MIME type image/png.

Since then, MNG support continues to be very low while APNG has become widely supported and, to keep the specification aligned with the reality of image use, the specification needs to be updated.

Also since then, a vendor-specific MIME type image/vnd.mozilla.apng was registered and later, an unregistered MIME type image/apng is used by browsers.

So, once a PNG specification incorporates APNG, then the image/apng should be registered with IANA.

This would also allow libpng to add support for APNG, rather than the current situation with many unofficial patches.

[svgeesus]

WikiMedia Commons has a useful diagram showing three PNG combined into one APNG. The license allows reuse, redistribution and remixing (with attribution).

[svgeesus]

Now added to spec:

• APNG Frame Based Animation
• Animation chunks

[svgeesus]

The image/apng registration was published by IANA today.