MDN's explanation of Vary is much better: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Va.... This article uses the phrase "the server", when it should really be "the cache server". The actual server is the one that actually sends that header. The way this post words it makes it seem like the browser sends the header ("you can simply use the Vary header to tell the server"), which would be pointless.