https://drafts.csswg.org/css-display/#transformations

The root element’s display type is always blockified.

https://drafts.csswg.org/css-display/#initial-containing-block

initial containing block
The containing block of the root element.

The term "root element" is neither hyperlinked nor explicitly defined. My skimming of the DOM spec is that, while it defines tree, root, and element, the root of a normal DOM tree would instead be the Document node. Nor does DOM appear to define how its definition of "root" would/could be modified by the adjective "element".

Searching further, it appears to me that document element would be the correct term to use in these cases.