In one sense, Assembly does have a runtime, provided by the kernel's system call interface, not to mention all the magic that the loader has to do to make libra

In one sense, Assembly does have a runtime, provided by the kernel's system call interface, not to mention all the magic that the loader has to do to make libraries work. Even the kernel has a runtime, in a less strict sense, provided by the BIOS and whatever CPU interrupts support it. The only thing that a program can do without any "runtime" at all is warm up your desk.