I didn't mean it would be physically impossible, which is hopefully implied, I mean, it would be de-facto impossible. Absent the perverse forces of anticompetitive behavior, browsers don't really have a good incentive to diminish the open nature of web standards by doing partnerships that bypass standards altogether. If you are not affiliated with Google and there is a healthy ecosystem of browsers, you just simply can tell them to bug off if they want some web feature you feel wouldn't be good for the health of the web. The interaction between browser vendors and certificate authorities has traditionally been a great example of how things can work out between different entities in an ecosystem, though outside Mozilla I am guessing most of the browser vendors are also CAs (but still have very little to no incentive to compromise or weaken the system.)

Meanwhile, in our current reality, both Google and Apple have or currently are shoehorning platform level attestation into the web in various different ways, something they are mostly able to do because they have so much control over multiple major ecosystems (among platforms, browsers, web services.) Mostly, even making them "standards", which would be hilarious if it wasn't literally evil. (Apple's approach to sneaking this in is innovative, in that it technically is a hardware platform attestation mechanism, but it was sold and initially implemented as a convenience feature. That and the underlying PAT technology can be used in strictly non-evil ways, like Kagi's rather clever application.)

It's a lot of words to say that I didn't mean literally impossible, but if we're going to get pedantic then a lot of words it is.