And since you won't be able to modify web pages, it will also mean the end of customization, either for looks (ie. DarkReader, Stylus), conveniance (ie. Tampermonkey) or accessibility.
The concept of having third party "trusted" attesters just screams abuse, corruption, and data selling. What would their business model be? Direct payments from browser companies? Selling "anonymized" data? What if their data security is crap it just ends up being another vector by which leaks and scams can happen? I doubt the average web user is going to understand that all of their data is going to be going thru some man in the middle when they Google something
What about competition? What if attester A has major ties with Microsoft edge and decide to block Firefox users? Or what about smaller third party apps that are made for accessibility reasons? Or hell - what about startups that are trying to enter the market?
The only reason these attesters would be trusted is because the large companies say they are. Theyd be completely at the whims of Google and Microsoft. What a stupid fucking idea with little upside for the end user