--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.

Output method xml

Serializing an HTML document with method="xml" writes XML where a browser expects HTML: an empty element comes out <br/> rather than <br>, a script or style body is escaped, and no <!DOCTYPE html> is emitted. Switch to method="html" when the document being built is HTML.

Incorrect:

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:output method="xml"/>
  <xsl:template match="/">
    <html><body><xsl:value-of select="."/></body></html>
  </xsl:template>
</xsl:stylesheet>

Correct:

<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:output method="html"/>
  <xsl:template match="/">
    <html><body><xsl:value-of select="."/></body></html>
  </xsl:template>
</xsl:stylesheet>

Only the outermost element the root template builds decides it, so an HTML fragment inside an XML document, an html in the XHTML namespace, and a named xsl:output are left alone.