Mr. Calvano with a really good explanation of HTTP Caching. Particularly interesting to me was the bit about the heuristic cache limits of different browsers—something I had never really dug into.
In the Chromium source as well as the Webkit source, we can see that the lifetimes.freshness variable is set to 10% of the time since the object was last modified (provided that a Last-Modified header was returned). The 10% since last modified heuristic is the same as the suggestion from the RFC. When we look at the Firefox source code, we can see that the same calculation is used, however Firefox uses the std:min() C++ function to select the less of the 10% calculation or 1 week.