What do you mean? Every single web view on iOS uses Safari's renderer. It's against the App Store rules to have your own renderer. The problem is that sure, if you design a website around Safari it'll work everywhere else, but it's a pain in the ass to design it to a 5 year old standard when all the other major browsers support other upgrades, extensions and capabilities that can make code easier/faster/better. It's most apparent when an open standard has replaced an Apple designed one that's inferior, and Apple refuses to change, such as WebSQL/IndexedDB.
Like I/E used to force you to do. Write for them and then write for everybody else that was current.