The url element holds the unique resource locator (URL) of an electronic version of the publication or of electronic information relevant to that publication.
<pubinfo> <pubdate type="primary"> <date> <year>1999</year> </date> </pubdate> <city>Sebastopol</city> <publisher>O'Reilly</publisher> <serial>1-56592-580-7</serial> <address>millera@dinosaurs.upenn.edu</address> <url>http://www.dinosaurs.upenn.edu/</url> </pubinfo>