If the server sends an "Expires" header in the response, then the client doesn't even need to do that check. With an expires header, the server has effectively

If the server sends an "Expires" header in the response, then the client doesn't even need to do that check. With an expires header, the server has effectively told the client that the data wont change until at least a particular date, and so the client honours that information. Last-Modified/If-Modified-Since is an optimisation trick which exists for the situation where the person running the website hasn't bothered to explicitly define expiry periods for content.