What about implementations? libpng seems pretty dead, 1.7 has been in development forever but 1.6 is still considered the stable version. Is there a current "canonical" png C/C++ library?
I mean, if the spec has been stable for two decades then maybe there just hasn't been much to fix? Especially since PNG is a relatively simple image format.
For modern C/C++ PNG implementations, consider lodepng (header-only), stb_image/stb_image_write (single-file), or libspng (active fork focused on performance and security) as more actively maintained alternatives to libpng.