I see various problems with the Becoming a formatting context section.

In some circumstances (See [CSS-CONTAIN-1] or [CSS3-MULTICOL] for examples), an element may need to become a formatting context.

"See" should be lowercase.

Situations that CSS2.1 said produced a block formatting context when in fact they only needed to become a formatting context should be listed here. This includes out-of-flow boxes, and boxes to which overflow applies and is different than visible. Maybe also mention block containers that are not block boxes.

If the element already establishes a formatting context of any kind, this condition is satisfied.

Except for inline formatting contexts! Inline formatting contexts are not contained at all, they are e.g. affected by floats, so contain: layout is not satisfied by a block container that establishes an inline formatting context, it must establish a block formatting context instead.

And what about ruby formatting contexts? I know very little about them, but I don't think establishing a ruby formatting context is enough neither.

I propose:

• If the inner display type is flow-root, table, flex or grid, the condition is satisfied.
• If the inner display type is flow, it is changed to flow-root.
• An element with a ruby inner display type always (not just if the outer display type is different than inline) generates two boxes: a principal box with the inner display type set to flow and the outer display type set to the outer display type of the element, and an inline-level ruby container. "Becoming a formatting context" only affects the principal box, changing its inner display type to flow-root.
• Not sure about layout internal types.