XSL is both a transformation language and a formatting language. The XSLT transformation part lets you scan through a document's structure and rearrange its content any way you like. You can write out the content using a different set of XML tags, and generate text as needed. For example, you can scan through a document to locate all headings and then insert a generated table of contents at the beginning of the document, at the same time writing out the content marked up as HTML. XSL is also a rich formatting language, letting you apply typesetting controls to all components of your output. With a good formatting backend, it is capable of producing high quality printed pages.
An XSL stylesheet is written using XML syntax, and is itself a well-formed XML document. That makes the basic syntax familiar, and enables an XML processor to check for basic syntax errors. The stylesheet instructions use special element names, which typically begin with xsl: to distinguish them from any XML tags you want to appear in the output. The XSL namespace is identified at the top of the stylesheet file. As with other XML, any XSL elements that are not empty will require a closing tag. And some XSL elements have specific attributes that control their behavior. It helps to keep a good XSL reference book handy.
Here is an example of a simple XSL stylesheet applied to a simple XML file to generate HTML output.
<?xml version="1.0"?> <document> <title>Using a mouse</title> <para>It's easy to use a mouse. Just roll it around and click the buttons.</para> </document>
<?xml version='1.0'?>
<xsl:stylesheet
xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version='1.0'>
<xsl:output method="html"/>
<xsl:template match="document">
<HTML><HEAD><TITLE>
<xsl:value-of select="./title"/>
</TITLE>
</HEAD>
<BODY>
<xsl:apply-templates/>
</BODY>
</HTML>
</xsl:template>
<xsl:template match="title">
<H1><xsl:apply-templates/></H1>
</xsl:template>
<xsl:template match="para">
<P><xsl:apply-templates/></P>
</xsl:template>
</xsl:stylesheet>