Make it an option for developers. They should be able choose between an OS-provided "stable" webkit, or the Safari-provided "evergreen" webkit. You could even

Make it an option for developers. They should be able choose between an OS-provided "stable" webkit, or the Safari-provided "evergreen" webkit. You could even make it so that the stable option is automatically used as a fallback if Safari isn't installed, if that ever becomes an option.