> What does that achieve?

Because then a user who wants to see the HDR image in all its full glory can do so. If the base image is not HDR, then there is nothing they can do about it.

> And why should "don't physically hurt my eyes" be an opt-in setting anyway instead of just the default?

While I very much support more HDR in the online world, I fully agree with you here.

However, I suspect the reason will boil down to what it usually does: almost no users change the default settings ever. And so, any default which goes the other way will invariably lead to a ton of support cases of "why doesn't this work".

However, web browsers are dark-mode aware, they could be HDR aware and do what you prefer based on that.