But functionally, this means that two renderers would have different outputs for the same file. It's not so bad when it's just HDR, but if this continues to grow, it WILL get bad. We've seen it many, many times before with hacked-on file extensions.

If it were Microsoft doing this (it's not), it would be just yet another embrace-extend-extinguish effort. But it's not. I have no doubt this author has good intentions, and this is a cool hack, but I think it will lead to confusion and poor outcomes in widespread usage.

It's really not great when an image format looks "somewhat" the same between renderers. In fact, I think that's worse than it not working at all. From experience, it leads to these situations where the same image can fragment over time just by virtue of being worked on by different people in different environments (apps, OSes, browsers, devices), and there's not really a clear way to tell which is which without extensively analyzing the metadata.

If Joe starts work on a HDR version, sends it to Sarah, Sarah adds a bunch of new edits but then saves it in an app that recompresses the image and overwrites the HDR, Joe might never know. And then if the format continues to expand, maybe a bit later Jane and Janet each add mutually incompatible extensions to the file without knowing it. The "base" image might look the same to all of these people, but each had specific edits in mind that the others might not have known about, because their renderers LOOKED like it was rendering the same image when it wasn't.

This isn't a hypothetical... it happened a lot when PNG first came out, and with PSD (Photoshop files) for a good decade or two until the subscription model came out.