I only just discovered that the current version of Safari (v6.1) under Mountain Lion includes the "Use Per-Tab Web Process" option that is enabled by default under Mavericks. You need to enable the Debug menu (instructions here) and it's the option at the top. What I'm curious to know is whether anyone has been running Safari with this option enabled for any length of time? Personally ever since the update to v6.1 I've found Safari to be very unresponsive, with webprocess frequently leaping to 100% CPU usage and causing all tabs to stop responding, so I'm hoping a process per tab may alleviate this issue, even if it doesn't run quite as well as it does under Mavericks (thanks to App Nap and other features). Of course the feature may also introduce instability, but I've only just enabled it so I can't really say. I'm going to give it a try for a while and add my findings.