I think OP means that the collection of visited prefixes can be indicative. If foo.example.com, bar.example.com, and baz.example.com are "undesirable" sites, an

I think OP means that the collection of visited prefixes can be indicative. If foo.example.com, bar.example.com, and baz.example.com are "undesirable" sites, and you have requested hash prefixes matching those for foo.example.com/index, foo.example.com/favicon, bar.example.com/index, bar.example.com/favico, baz.example.com/index, and baz.example.com/favicon, you might be able to start making educated guesses that the user is visiting such sites. It really depends how much of an overlap for "acceptable" browsing falling into these buckets there is, or if you can weight certain prefixes as likely to be indicative of visiting "undesirable" sites. You might not able to say this user visited this specific page at this specific time, but you might be able to fingerprint a user as visiting a particular category of sites. Furthermore, I'm unclear how exactly Safe Browsing is implemented, but if it checks every resource on a page, the set of prefixes for all resources on a given page may leak more data than one query on its own, so you can look for requests for that set of queries in a short time frame to identify a particular page.