Comment by Joker_vD

See, the caching mechanism in HTTP/1.1 is actually quite nicely designed in the sense that you can completely ignore it: don't look at any caching-related fields, always pass the requests up and the responses down — and it will be correct behaviour. But when you start to implement caching, well, you have to implement it, even the inconvenient but mandatory parts.

Yes, I know (and personally experienced) that most clients forget to include "Vary" field, or don't know that it exists, or the problem that this header solves exists) but when they do include it, they actually mean it and rely on it being honoured, one way or another.

PS. By the way, 7230 is obsoleted, you're supposed to use 9112 now.