[css-cascade] "document order" is undefined

[cvrebert]

• https://drafts.csswg.org/css-cascade/#cascade-order
• https://drafts.csswg.org/css-cascade-5/#cascade-order
• https://drafts.csswg.org/css-cascade-6/#cascade-order

Quoting from this section (emphasis added):

Order of Appearance
The last declaration in document order wins. For this purpose:

• [...]
• Declarations from style attributes are ordered according to the document order of the element the style attribute appears on, and are all placed after any style sheets. [CSSSTYLEATTR]

Presumably "document order" refers to https://www.w3.org/TR/DOM-Level-3-Core/glossary.html#dt-document-order and should be hyperlinked thereto.
However, that spec is obsolete nowadays. I think y'all want "tree order" instead.

E.g. https://drafts.csswg.org/cssom/#css-style-sheet-collections already refers (with an extra level of unconditional indirection thru WHATWG HTML, for some reason) to WHATWG DOM's "tree order":

[...] Any CSS style sheets associated with the DocumentOrShadowRoot, in tree order

A grep of the CSS2 spec plaintext turned up references to "document order", but apparently it likewise neither defines the term nor references another spec for it.

[Loirooriol]

It's defined in https://drafts.csswg.org/css-display-3/#document-order

The order in which boxes or content occurs in the document (which can be different from the order in which it appears for rendering). For the purpose of determining the relative order of pseudo-elements, the box-tree order is used, see CSS Pseudo-Elements 4 § 4 Tree-Abiding Pseudo-elements.

[fantasai]

Probably should be cross-linking to that definition, but also it doesn't cover what document order means for style sheets, so we either need to define that or create a new term for style sheet order (“source order” if we want to be generic?).