[k__]

So they can but don't have to be marked?

[taejo]

Yes. Many programs rely on W&X (I believe the JVM is a prominent example)

[masklinn]

Any JIT-based system requires executable writable memory. This includes the JVM, LuaJIT, Pypy, HHVM and all modern browsers.

[m_eiman]

It'd be quite possible for a JIT to have the memory first writable but not executable when creating the code, then the other way around when running it. No need to be both at the same time.