The Node.lookupNamespaceURI() method takes a prefix and returns the namespace URI associated with it on the given node if found (and null if not). Supplying null for the prefix will return the default namespace.
Due to bug 312019, this method does not work with dynamically assigned namespaces (i.e., those set with Node.prefix).
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https://developer.mozilla.org/en-US/docs/Web/API/Node/lookupNamespaceURI