Safari doesn't lag that far behind WebKit and iOS/OSX both have very frequent update cycles. And has WebKit really been that bad at adopting internet standards ?
Sadly yes. While it's not really as horrible as many people (myself included) have said, the problem is their release cycle, and iOS's lock in.
One a year means that it will probably be 2 years before any new features are "stable". (for example IndexedDB support came out and it was so buggy that it was unusable, it wasn't until ios8 that it was even somewhat usable, and it still has several game-breaking bugs)
So you run into problems like this where in 6 months every other major browser will have stable shadow DOM support, and safari will still be at least 6 months away from its first (possibly buggy) release, meaning there could be another 12 month wait after that to get a good implementation. And while any other platform can install an alternative iOS users are stuck with safari, and there is literally no way around it.
One a year means that it will probably be 2 years before any new features are "stable". (for example IndexedDB support came out and it was so buggy that it was unusable, it wasn't until ios8 that it was even somewhat usable, and it still has several game-breaking bugs)
So you run into problems like this where in 6 months every other major browser will have stable shadow DOM support, and safari will still be at least 6 months away from its first (possibly buggy) release, meaning there could be another 12 month wait after that to get a good implementation. And while any other platform can install an alternative iOS users are stuck with safari, and there is literally no way around it.