Becoming a formatting context is defined as follows:

If the element already establishes a formatting context of any kind, this condition is satisfied. Otherwise, it is made to establish a BFC by changing its inner display type to flow-root.

But what happens if an element that does not have inner nor outer display types (i.e. it has display: none or display: contents) becomes a formatting context? Such an element won't establish a formatting context, so its inner display type should be changed to flow-root, but it doesn't have an inner display type!

I think "this condition is satisfied" should also apply in that case.