XSL Formatting Objects
| Filename extension | .xml |
|---|---|
| Internet media type |
application/xml,text/xml (deprecated) |
| Uniform Type Identifier (UTI) | public.xml |
| Developed by | World Wide Web Consortium |
| Type of format | Markup language |
| Contained by | XML |
| Standard | 1.1 |
XSL-FO (XSL Formatting Objects) is a markup language for XML document formatting that is most often used to generate PDF files. XSL-FO is part of XSL (Extensible Stylesheet Language), a set of W3C technologies designed for the transformation and formatting of XML data. The other parts of XSL are XSLT and XPath. Version 1.1 of XSL-FO was published in 2006.
XSL-FO is considered feature complete by W3C: the last update for the Working Draft was in January 2012, and its Working Group closed in November 2013.
Unlike the combination of HTML and CSS, XSL-FO is a unified presentational language. It has no semantic markup as this term is used in HTML. And, unlike CSS which modifies the default presentation of an external XML or HTML document, it stores all of the document's data within itself.
The general idea behind XSL-FO's use is that the user writes a document, not in FO, but in an XML language. XHTML, DocBook, and TEI are all possible examples. Then, the user obtains an XSLT transform, either by writing one themselves or by finding one for the document type in question. This XSLT transform converts the XML into XSL-FO.
Once the XSL-FO document is generated, it is then passed to an application called an FO processor. FO processors convert the XSL-FO document into something that is readable, printable or both. The most common output of XSL-FO is a PDF file or as PS, but some FO processors can output to other formats like RTF files or even just a window in the user's GUI displaying the sequence of pages and their contents.
The XSLT language itself was originally conceived only for this purpose; it is now in widespread use for more general XML transformations. This transformation step is taken so much for granted in XSL-FO that it is not uncommon for people to call the XSLT that turns XML into XSL-FO the actual XSL-FO document itself. Even tutorials on XSL-FO tend to be written with XSLT commands around the FO processing instructions.
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