The definition of what is a core feature and what is advanced functionality changes with time. When IE6 came out it was the most advanced browser available, including support for what was considered advanced functionalities back then. But over the long time IE6 existed expectations changed and things which were once advanced or not even thought of were now core. The same is happening with Safari, with the difference that thanks to Apples blocking of different browser engines people cannot just switch to another browser and light a fire under Apples ass.