ATX PSU must supply power for 16 ms after power off. It also supplies power good signal which can indicate that power is not available. So operating system has

ATX PSU must supply power for 16 ms after power off. It also supplies power good signal which can indicate that power is not available. So operating system has a chance to flush caches. Of course that's not few seconds, but should be enough for practical uses. I don't know whether popular operating systems implement this behaviour. Some server disks might have a backup power source, but that's certainly not a consumer hardware.