In 25 Words or Less

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XSLT is a standard dialect of XML used to define transformation of an XML input to an output XML (or HTML or text).

In More Words

An XSLT file is an XML written according to the XSLT standard (defined by the W3C group along with many other XML dialects and XML itself). The XSLT file describes how to transform an XML input file into user-defined XML, HTML or text output. The purpose can be either to generate new XML or to produce a more human-readable form of the original XML.

All XSLT Elements and Attributes are defined in the XSLT namespace ("http://www.w3.org/1999/XSL/Transform"), which is typically assigned to the "xsl" prefix (see example below).

An XSLT engine — given the XSLT and XML input files — generates the desired output file. Also, many applications that can display XML (most browsers, for example), can use XSLT to present the XML as HTML.

The canonical basic usage is:

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
    <xsl:template match="/*">
        …main template XSLT “code” goes here…
    </xsl:template>
    
    …sub templates XSLT “code” goes here…
    
</xsl:stylesheet>