It's a combination of different headers that's hard to sum up in a short comment. A good article on the subject should talk about all these headers: Expires,

It's a combination of different headers that's hard to sum up in a short comment. A good article on the subject should talk about all these headers: Expires, Cache-control, Etag, Pragma, Vary, Last-Modified Key CDN has an article on it. They certainly would have experience and expertise there. I didn't read the whole thing, but it seems to have it covered: https://www.keycdn.com/blog/http-cache-headers https://www.keycdn.com/blog/http-cache-headers There's also some interesting exceptions where rules aren't followed. Like browsers typically have a completely separate cache for favicons. I suppose because they use the icons in funny/different ways, like bookmarks. There are also sometimes proxies (especially corporate MITM ones) that don't follow the rules. Hence the popularity of cache-busting parameters like you described.