Presumably, the most common method of citing this document is by URL; use the first one appearing above under the heading "This Version".
A correct bibliographic reference, for use in paper publications, would be:
"Extensible Markup Language (XML) 1.0", Tim Bray, Jean Paoli, and C. M. Sperberg-McQueen, 10 February 1998. Available at http://www.w3.org/TR/REC-xml
Back-link to specCopyright © 1998, Tim Bray. All rights reserved.