wow Safari is the IE5 of browsers now, having to do so many work arounds like we used to have to do with IE5 and now its killing hardware that you can't even replace.
It’s only “behind” relative to Chrome, the dominant browser which more or less sets the standards these days. Calling Safari “behind” is judging browsers by how similar to Chrome they are.
Also Firefox and even Edge when the IE version was being developed. They've also refused to implement features which would bring web apps in line with the the capabilities of native iOS apps (things like push notifications, offline support with data that doesn't get cleared every 7 days).