在 RFC 7234 里描述启发式新鲜度:
If the response has a Last-Modified header field (Section 2.2 of [RFC7232]), caches are encouraged to use a heuristic expiration value that is no more than some fraction of the interval since that time. A typical setting of this fraction might be 10%.
应该是说如果过有 Last-Modified 这个头,那么启发式新鲜度的时间一般取 Last-Modified 的10%,而不是有 Last-Modified 才会使用 启发式新鲜度 来设置过期时间。