The CSSNamespaceRule.namespaceURI read-only property returns a DOMString containing the text of the URI of the given namespace.

API CSSNamespaceRule CSSOM namespaceURI Property Reference

The CSSNamespaceRule.prefix read-only property returns a DOMString with the name of the prefix associated to this namespace. If there is no such prefix, returns  null.

API CSSNamespaceRule CSSOM prefix Property Reference