In computing, FOSI is a stylesheet language for SGML and, later, XML. A style sheet language is a Computer language used to describe the presentation of Structured documents. The Standard Generalized Markup Language ( ISO 88791986 SGML) is an ISO Standard Metalanguage in which one can define Markup languages Don't change "Extensible" It stands for Formatting Output Specification Instance. FOSI was developed by the US Department of Defense to control the pagination and layout of SGML and XML technical data. The United States Department of Defense ( DOD or DoD) is the federal department charged with coordinating and supervising all agencies and functions of the government FOSI stylesheets are themselves written in SGML, an approach that would later be adopted by XSL. In Computing, the Extensible Stylesheet Language ( XSL) a family of Transformation languages allows one to describe how to format or transform files encoded FOSI was implemented by, among others, Arbortext and X. Arbortext is an XML -based publishing system available from PTC. Systems.