Sorry, but no way.
To violate privacy (aka enable tracking) a sub-iFrame could be set up that uses "local storage" with a parent page security policy that allows communication across the iFrame boundary. Sorry, yes, I am being a bit vague.
Who cleans up ~/Library/Safari/Databases? I personally see crud in this directory from 2011 that has been migrated from older systems.
Almost not relevant now, but Flash also had a "local storage" system that was shared across all Flash Apps. It also allowed (before sandboxing) local apps to proxy and communicate (via shared memory) with any standalone Flash App on the system through any page that used the Flash plugin -- i.e any running web browser, violating all attempts to have web compartmentalization rules.