The way If-None-Match is that the browser says "give me the latest if this ETag represents an out-of-date resource, otherwise I'll keep using my copy." It's no

The way If-None-Match is that the browser says "give me the latest if this ETag represents an out-of-date resource, otherwise I'll keep using my copy." It's not clear to me how you're proposing this work instead? (Also, in many cases the server uses a hash of the inputs to generating the resource, which isn't something externally verifiable)