The definition is a bit cryptic. A web document is defined by a technical language. We now don't use document much. Then you need a definition. From CSS definition.
Document is a tree of elements.
In WCAG the definition goes into the distinction between information and presentation. Positions to be put, discussion period, then vote. It makes sense to me to start with accessibility, althugh i ma not particularly concerned - I think we have a strong document, whatever the order JB Agree with Gregory that accessibilty is a part of the core design and should come first, but having the interface accessibility first could increase confusion about what is in the rest of the document. Disagree with keeping it in first position to prove a point. Also disagree strongly that it is not important to address accessibility in these guidelines - I would ahve an extreme problem if we did not address that problem.