--fix-suggestions rewrites this one, and a plain --fix leaves it alone: the correction changes what the stylesheet does, or is one of several the check would accept.

Incorrect use of boolean constants

When an xsl:if or xsl:when test is the bare string 'true' or 'false', it is a non-empty string, so the branch is always taken — test="'false'" runs when you meant it never should. Say what you mean with the boolean functions true() and false().

Incorrect (the branch always runs):

<xsl:if test="'false'">
  <xsl:value-of select="."/>
</xsl:if>

Correct:

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

The quote makes no difference: 'true' and "true" are the same string, and an attribute already standing in double quotes writes the constant test=""true"", which is always true exactly as the single-quoted spelling is.

Comparing a value to the string 'true' is a different thing and correct — XML attributes are strings, so @active = 'true' is how you test one. That, and literal output like <td hidden="true"/>, are left alone.