Google used to do something called SDCH (Shared Dictionary Compression for HTTP), where a delta compression dictionary was downloaded to Chrome. The dictionary

Google used to do something called SDCH (Shared Dictionary Compression for HTTP), where a delta compression dictionary was downloaded to Chrome. The dictionary had to be updated from time to time to keep a good compression rate as the Google website changed over time. There was a whole protocol to handle verifying what dictionary the client had and such.