In a recent discussion #11126 (comment), it was stated that a block formatting context is established by a "block container" which isn't block-level. However, the initial containing block establishes a block formatting context even though it is not a block container—it’s defined as a rectangle. This suggests that a block formatting context can be established either by a block container or specifically by the ICB.

Could the documentation clarify exactly what types establish a block formatting context? Specifically, it would be helpful to understand if both block containers and the ICB—or potentially other types—are capable of establishing this context.