If Apple wanted more web devs to support Safari they should port it to Linux and Windows. The web is supposed to be an open standard, you shouldn't need a devices and software from a specific manufacturer to develop for it (I say that posting from a Mac).
But there isn't anymore, so there's no way for a web developer to ensure Safari compatibility (unless you expect every dev shop in the world to buy a Mac just for that purpose).
Web devs ignore Safari at their own risk, lest 100% of iPhone users be unable to use their site.