FO (Formatting Objects) is part of the w3c's XSL specification. It is like html in that it describes how content should be presented, but unlike html, fo focuses on the printed page, so it provides for headers, footers, page number, and so on.

	XSL is a language for expressing stylesheets. It consists of three parts: XSL Transformations (XSLT): a language for transforming XML documents, the XML Path Language (XPath), an expression language used by XSLT to access or refer to parts of an XML document. (XPath is also used by the XML Linking specification). The third part is XSL Formatting Objects: an XML vocabulary for specifying formatting semantics. An XSL stylesheet specifies the presentation of a class of XML documents by describing how an instance of the class is transformed into an XML document that uses the formatting vocabulary.
-- w3c

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