Currently,ResponseCacheStrategy will always remove theLast-Modified header when merging ESI responses. This makes it impossible to revalidate the result.

My suggestion is to keep the maximum = most recentLast-Modified value on the final response as long as all (sub-)responses providedLast-Modified.

Does that make sense? Why not? Am I missing edge cases?

Ping@dbu and@Toflar since I know you're familiar with HTTP Caching, and@aschempp since you wrote a good amount of the code involved.