Important! Having problems with clients not getting the latest
version of page updates? Here’s the solution. Make sure your server
is sending the Expires and Cache-Control headers. These will
indicate to the browser that it should send an HTTP request to
revalidate the cache entry when a user accesses the page. If it is
still valid, you still save bandwidth. Otherwise, you guarantee that
the user is seeing a fresh page. You can set this to zero to always
require revalidation every time the user visits the page.
20180728/DuckDuckGo apache cache expiration header
20180728/https://httpd.apache.org/docs/2.4/mod/mod_expires.html
20180728/http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.9