Expires
and Cache-Control
are "strong caching headers"
Last-Modified
and ETag
are "weak caching headers"
First the browser check Expires/Cache-Control
to determine whether or not to make a request to the server
If have to make a request, it will send Last-Modified/ETag
in the HTTP request. If the Etag
value of the document matches that, the server will send a 304 code instead of 200, and no content. The browser will load the contents from its cache.