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Do you remember when Facebook researched the efficiency of the browser’s cache? Patrick McManus now made a reference implementation in Firefox to improve the situation by adding a new directive in the Cache-Control- header: immutable. It indicates that the response body will not change over time and is complementary to the lifetime cachability expressed by max-age. Thus, the browser will assume that, if not expired, it’s unchanged and should not be revalidated. The goal is to land support in Firefox 49 and tests have shown that this will make a big difference.