It only works in Firefox Nightly. The flag exists in Beta and Stable, but since Stylo isn't compiled with these versions the flag doesn't actually do anything.
From what I can tell, Beta 56 has it (but you have to enable it), Nightly 57 has it (https://bugzilla.mozilla.org/show_bug.cgi?id=1330412 is tracking when it'll be enabled by default), and Stable 55 does not have it at all (although the pref might be there).