--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.

What makes the document HTML is its outermost element, not an html somewhere inside it. An XML document may embed an HTML fragment and stay XML — an Atom entry's content, an XHTML island in a larger vocabulary — and there method="xml" is the right serialization. An html in the XHTML namespace is a third case: XHTML serializes as xml in XSLT 1.0 and as xhtml from 2.0, never as 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>