The Edge Conference, which one can attend by invitation only, includes "delegates" from Google, Mozilla, Facebook, but not from Apple. Many of the web API's unsupported on Safari include functions provided by API's in iOS, or even Android. Some people want to create Web Apps, that create experiences very similar to iOS, but run on a mobile or desktop web browser. Apple would prefer you develop such Apps using iOS using Swift or Objective-C API's, which run natively with better performance in security. Why should Apple support API's on Safari, that allow Web apps to recreate the iOS experience on non-Apple devices? Why do you think Steve Jobs banned JAVA and Adobe Flash from iPhones?
BTW, you can see which web API's are supported on a given web browser by going to http://caniuse.com/