Everything new is optional. This is not a breaking change. Old PNGs and software continue to work just fine. And these new changes are backwards-compatible as m

Everything new is optional. This is not a breaking change. Old PNGs and software continue to work just fine. And these new changes are backwards-compatible as much as they can be. So old software can display a new PNG and be mostly correct. By that I mean, the user will still say "it is a picture of a red apple". But if the software isn't HDR, they might not get the bright highlights and inky blacks of the HDR PNG.