Incorrect use of boolean constants

The strings 'true' and 'false' are non-empty strings that always evaluate to boolean true. Use the XPath functions true() and false() instead.

Incorrect:

<xsl:if test="@active = 'true'">
  <xsl:value-of select="."/>
</xsl:if>

Correct:

<xsl:if test="@active = true()">
  <xsl:value-of select="."/>
</xsl:if>