If the remote server complies, we can optimize cache re-validation with conditional headers ([if-none-match](https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/If-None-Match), [if-modifed-since](https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/If-Modified-Since)).
If the remote server complies, we can optimize cache re-validation with conditional headers (if-none-match, if-modifed-since).