PNG had more focus on backward and forward compatibility, but the fact that PNG can be "extended" in this way is not that unusual for file formats composed of multiple sections (chunks in PNG). Especially considering that other aspects of PNG effectively failed, for example it is technically possible to add new compression method or color type to IHDR, but that would give you a file completely unreadable by existing decoders. CPNG essentially works by reinterpreting PNG in a different way when certain conditions are met.