Document#lookupNamespaceURI / Document#lookupPrefix are not implemented according to spec #213
Description
The specs (https://www.w3.org/TR/DOM-Level-3-Core/namespaces-algorithms.html#lookupNamespaceURIAlgo) defined Document#lookupNamespaceURI to use the documentElement for the actual lookup.
The way this method is implemented in the xmldom implementation uses the _nsMap of a document, which is undefined.
The same with attribute nodes, they should look up using the ownerElement, not ownerDocument.