The XML FAQ

Right here: Extensible Markup Language (XML) 1.0 (Bray et al, 4 February 2004) (http://www.w3.org/TR/REC-xml). Includes the EBNF, and all the normative material. There are also versions in Japanese; Spanish; Korean; a Java-ised annotated version, and Bob DuCharme's book, XML: The Annotated Specification.

Eve Maler maintains the DTD used for the spec itself; the DTD is also to encode several other W3C specifications, such as XLink, XPointer, DOM, XML Schema, etc. There is documentation available for the DTD. Note that the XML spec needs to use a special one-off version of the DTD, since the real original DTD used for it has long since been lost.