Doesn't Apple use Webkit, same as Google? Isn't that functionally the same, except for Google's optimizations?
I'm not sure how close to each other these rendering engines are. I imagine they're still very very similar, but now that the fork has occurred, time will tell how divergent (or not) they become.
Check this out for a good summary: https://www.youtube.com/watch?v=dQw4w9WgXcQ