I'm bit confused with this header. I already added "Expires" and "Cache-Control: max-age" in my .httaccess. I also removed "E-tag" and "Last-Modified" as per advice:
If you remove the Last-Modified and ETag header, you will totally eliminate If-Modified-Since and If-None-Match requests and their 304 Not Modified Responses, so a file will stay cached without checking for updates until the Expires header indicates new content is available!
so why Google's PageSpeed shows that?:
Specify a cache validator: The following resources are missing a cache validator. Resources that do not specify a cache validator cannot be refreshed efficiently. Specify a Last-Modified or ETag header to enable cache validation for the following resources
