The HTTP header If-None-Match is mentioned in your links, I don't see the problem with that, except ETags has their problems. https://developer.mozilla.org/en-

The HTTP header If-None-Match is mentioned in your links, I don't see the problem with that, except ETags has their problems. https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If-None-Match https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/If...