CSS 2.2 Section 9.3 says that the root element is out-of-flow. CSS-Display's (convenience) Glossary definition of out-of-flow does not mention the root element.

Sometimes the root element is treated like it's out-of-flow, e.g. by default, its box contains its floats, showing that it establishes a BFC (2.2 Section 10.6.7) although from (2.2 Section 9.4.1) it's not clear why. The only clause which could fit is "block containers that are not block boxes". It's definitely by default a block container, so for it not to be a block box, it would have to be not block-level. Which is possible if the initial containing block doesn't establish a BFC, . (I can't find any CSS spec that says anything about this), since only a box that participates in a BFC can be block-level (2.2 Section 9.2.1)

On the other hand, for the calculations of the widths and heights, the root element's box is treated according to the rules of Block-level, non-replaced elements in normal flow (2.2 Sections 10.3.3 & 10.6.3).

I think 10.3.3. and 10.6.3 definition should be extended to make clear that they apply to the root element.