if a variable has some defined value : variable « XSLT stylesheet « XML Tutorial

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2. XSLT stylesheet
3. variable

File: Data.xml
<?xml version="1.0" encoding="utf-8"?>
<data>
  <animal>cat</animal>
  <animal>dog</animal>
  <animal>cow</animal>
</data>
File: Transform.xslt

<?xml version="1.0" encoding="utf-8"?>
<xsl:stylesheet
      version="1.0"
      xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
    <xsl:variable name="color">red</xsl:variable>
    <xsl:template match="//animal">
      <xsl:choose>
        <xsl:when test="boolean($color)">
          <P style="color:{$color}">
            <xsl:value-of select="."/>
          </P>
        </xsl:when>
        <xsl:otherwise>
          <paragraph>
            <xsl:value-of select="."/>
          </P>
        </xsl:otherwise>
      </xsl:choose>
    </xsl:template>
</xsl:stylesheet>

Output:

<?xml version="1.0" encoding="UTF-8"?>
  <P style="color:red">cat</P>
  <P style="color:red">dog</P>
  <P style="color:red">cow</P>