Well that's what you do, via the HTML. But the concern is that CSP treats the HTML served as untrusted. You could fix it by asking the server, but at that point

Well that's what you do, via the HTML. But the concern is that CSP treats the HTML served as untrusted. You could fix it by asking the server, but at that point, you're making a request to the server for the resource, killing some of the point of caching. And it seems wrong to have "if-not-hash" headers as part of this proposal; that'd be better off as an improvement to the HTTP caching stuff overall. But putting the verified hashes in the HTTP headers is fine as CSP already relies on the headers having integrity.