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

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