From that article: "We expect to significantly reduce the number of prefixed properties supported over time but Web compatibility will require us to keep around

From that article: "We expect to significantly reduce the number of prefixed properties supported over time but Web compatibility will require us to keep around prefixed versions of some features." The features WebKit needs to keep around to continue to work with many websites are presumably about the same as those Firefox felt it had to implement to work with many websites. (Disclaimer: I work on Firefox.)