Dominique Haza&#235;l-Massieux; Lynne Rosenthal. <a href="http://www.w3.org/TR/2005/NOTE-spec-variability-20050831"><cite>Variability in Specifications.</cite></a> 31 August 2005. W3C Note. URL: <a href="http://www.w3.org/TR/2005/NOTE-spec-variability-20050831">http://www.w3.org/TR/2005/NOTE-spec-variability-20050831</a>