Safari does not run at ring zero. However, it can make system calls into the ring-zero kernel and drivers. If there's a bug in the kernel or drivers, that can

Safari does not run at ring zero. However, it can make system calls into the ring-zero kernel and drivers. If there's a bug in the kernel or drivers, that can cause the kernel to crash. There are other scenarios that don't involve a ring-zero vulnerability but still result in a "reboot". Safari coordinates with other system processes, like the one that shows the home screen. If a web site can cause Safari to cause that process to crash, it would feel like a "reboot", even though no ring-zero code was attacked.