Anything that deals strictly with the W3C DOM.
For example, anything dealing the DOM2 Core, Events, Style/CSS, Traversal/Range, SVG DOM, (future?) MathML DOM, implementations of the DOM3 Working Draft, etc would be posted here.
"My DOM3 XPath Expression does not evaluate to all the nodes it should"
Gecko has implemented the working draft of the DOM3 XPath module, and any questions on that would be posted here. If it was just a question on XPath itself, it would go to the XML forum.
Another example, "How do I fire an event in NS7?"
NS7 uses the DOM2 Events module, and would best be posted here.
Maybe more common posts though would be about just the DOM1 Core, something I can imagine thay be typical:
"document.body.firstChild in IE is different from document.body.firstChild in NS"
And that's because Gecko treats indentation as a text node...