This is mostly a precursor to the attribute nodes implementation for IE9+ modes later, since most of them are stubs in legacy modes. There's also no way to insert them in legacy modes via node props either, since they don't expose IHTMLDOMNode* interfaces.
-- v2: mshtml: Implement removeChild for attributes. mshtml: Implement replaceChild for attributes. mshtml: Implement nextSibling prop for attributes. mshtml: Implement previousSibling prop for attributes. mshtml: Implement lastChild prop for attributes. mshtml: Implement firstChild prop for attributes. mshtml: Implement childNodes prop for attributes. mshtml: Implement hasChildNodes for attributes. mshtml: Implement insertBefore for attributes. mshtml: Implement appendChild for attributes. mshtml: Implement cloneNode for attributes. mshtml: Implement ownerDocument prop for attributes. mshtml: Implement attributes prop for attributes. mshtml: Implement nodeType prop for attributes.