[mjn]

The PNG group's noninclusion of APNG support in libpng isn't entirely due to simplicity, but partly to philosophical disagreements: they consider single-image and multiple-image formats inherently different, and believe they should be supported separately rather than mashed together. Many of the same group officially developed and support MNG as the officially standardized multi-image format related to PNG. And they do have a reference implementation of that, libmng: http://libmng.com/

Alas, both have stalled: MNG was initially implemented by Mozilla but later dropped, and the PNG/MNG folks consider APNG a hackish substitute so are unlikely to standardize or support it.