Lesson 9 — Transforming XML

XSLT Basics

XSLT (Extensible Stylesheet Language Transformations) transforms an XML document into another format entirely — often HTML for display, but also plain text or a different XML structure.

A Simple Transformation

An XSLT stylesheet is itself an XML document containing template rules that match parts of the source document and specify what output to generate for each match, often using XPath to select the relevant data.

XSLT — Transforming a book list to HTML 
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:template match="/library">
    <ul>
      <xsl:for-each select="book">
        <li><xsl:value-of select="title"/></li>
      </xsl:for-each>
    </ul>
  </xsl:template>
</xsl:stylesheet>

Common Mistakes

  • Forgetting that XSLT stylesheets are themselves XML documents and must follow XML's own well-formedness rules.
  • Using an incorrect XPath expression inside xsl:value-of or xsl:for-each, causing the transformation to output nothing.
  • Assuming XSLT can only output HTML — it can transform XML into virtually any text-based format, including other XML.
  • Not testing a transformation against multiple sample input documents, missing edge cases with optional or repeated elements.

Professional Tip

XSLT relies heavily on XPath expressions to select the data it transforms, so a solid grasp of XPath (the previous lesson) makes learning XSLT considerably more approachable.

Your Turn

Extend the XSLT example above to also include each book's author next to its title in the generated HTML list.

Mini Quiz

What is the primary purpose of XSLT?