[css-display] The principal box is unique

[Loirooriol]

From 6ed5a55, https://drafts.csswg.org/css-display/#intro says

Typically, an element generates a single <a>principal box</a>

This seems to imply that in some cases there may be more than one principal box.

This contradicts https://drafts.csswg.org/css-display/#principal-box:

When an element generates one or more boxes, one of them is the principal box

And my understanding was that the uniqueness was ratified in #1604

Instead, I would say

Typically, an element generates a single <a>box</a>, the <a>principal box</a>, which represents the element and contains its contents in the <a>box tree</a>.

[fantasai]

Fixed.