The meaning of "the Window object's newest Document object" isn't immediately obvious, IMO, and the fact that #initialise-the-document-object says "change the document attribute of the Window object" also feels weird. Maybe something more like...

# Window objects have an associated **current document**.
#
# Note: the _current document_ associated with a Window can change in
# exactly one case, …about:blank….

(near #the-window-object)

# The **document** IDL attribute must return the Window object's
# _current document_.

(near #dom-document-0)

# If the browsing context's only entry in its session history is the
# about:blank Document that was added when the browsing context was
# created, and navigation is occurring with replacement enabled, and
# that Document has the same origin as the new Document, then change
# the _current document_ of the Window object of that Document to
# the new Document.
#
# Otherwise, create a new Window object with the new Document as its
# _current document_, and associate it with the Document.

(near #initialise-the-document-object)

There's a _lot_ of prose that just says things along the lines of "The Window object of the Document object of...". I don't think we'll ever catch all of it. So at some level, this will always rely on an implicit association between a Document and a Window.

Is there some way you would be satisfied that this was clarified that didn't involve introducing a new term that doesn't get used everywhere?

Maybe just add something like

# Note: the Document object associated with a Window can change in
# exactly one case, …about:blank….

then.

Oh, yeah, that's totally doable. I have that kind of thing in various places. Happy to add that here too.

Checked in as WHATWG revision r8801.
Check-in comment: Add a note to clarify window.document, and fix some spellings
https://html5.org/tools/web-apps-tracker?from=8800&to=8801