> I’m guessing the primary use case for this will be setting cookies. There's a huge difference between "this could potentially be used to fingerprint a client

> I’m guessing the primary use case for this will be setting cookies. There's a huge difference between "this could potentially be used to fingerprint a client" and "this will primarily be used to fingerprint a client". There are many potential fingerprinting mechanisms that rely on tracking what the client already has in its cache. Clients could easily partition their cached dictionaries by origin, just as they could partition other cached information by origin, and that would prevent using this for cross-origin tracking. This proposal would help substantially with smaller requests, where a custom dictionary would provide better compression but not better enough to offset the full size of the dictionary.