In fact there is a new feature Chrome is championing (and just shipped) called "Compression dictionary transport" - https://datatracker.ietf.org/doc/draft-ietf-httpbis-compress... / https://chromestatus.com/feature/5124977788977152 that allows any HTTP resource to specify the dictionary it wants to use (including the "use me as the dictionary for future reuqests") which allows a website to use a dictionary that specialized to _its_ contents instead of the contents of something completely different.