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The entire web security model assumes we can trust browsers to implement web security policies!

I appreciate that, but in the case of TLS or CSRF tokens the server is not blindly trusting the browser in the way Sec-Fetch-Site makes it.

Sure it is. The same-origin rule that holds the whole web security model together is entirely a property of browser behavior.

That's indeed a good example of prior full trusting of the browser by the server.