Plan 9 from Bell Labs’s /usr/web/sources/contrib/fernan/nhc98/src/libraries/HaXml/tests/xml-conformance/sun/invalid/attr05.xml
<!DOCTYPE root [ <!ELEMENT root EMPTY> <!ATTLIST root token NMTOKEN #REQUIRED > <!-- tests the "name token' VC for an NMTOKEN value --> ]> <root token="dev@null"/>