Wouldn't this in practice be a lot like Passkeys? But it might be more difficult to integrate this kind of approach to the stacks we use, whereas Passkeys fits in relatively easily.

I suppose client cert would protect against from a MitM attack, if the client failed to notice it, or if the MitMer has the website keys to make a perfect attack.

Chrome would be more likely to implement a persistent and identifiable (to Google alone) tracking cookie replacement and ship it worldwide, which iirc they did — and then cancelled, of course. They seem to be focusing instead on improved tracking of Android users from the kernel up, rather than browsers from the headers down; GrapheneOS is, presumably, viewed as a serious threat to their advertising revenue.

https://privacysandbox.google.com/blog/update-on-plans-for-p...