The current definition of "block container" in CSS Display Module Level 4 says:

"A block container either contains only inline-level boxes participating in an inline formatting context, or contains only block-level boxes participating in a block formatting context (possibly generating anonymous block boxes to ensure this constraint, as defined in CSS2§9.2.1.1)."

This phrasing feels slightly circular, as it defines a "block container" by simply referring to "a block container," without immediately stating what it is (a type of box).

I suggest rewording it slightly for clarity, such as:

"A block container is a box that either contains only inline-level boxes participating in an inline formatting context, or contains only block-level boxes participating in a block formatting context (possibly generating anonymous block boxes to ensure this constraint, as defined in CSS2§9.2.1.1)."

This makes it immediately clear that a block container is a box, improving the definition's clarity without altering its meaning.