--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.

Missing version in stylesheet

Every stylesheet declares the XSLT version it conforms to, and one that declares none is invalid — a conformant processor is required to reject it. Which attribute carries the declaration depends on the root element.

A root XSLT defines itself carries a plain version: xsl:stylesheet, its exact synonym xsl:transform, and — since XSLT 3.0 — xsl:package, which declares a module other stylesheets can use. A simplified stylesheet — one whose root is a literal result element and whose whole transformation is that element's content — carries xsl:version instead, in the XSLT namespace. An unprefixed version will not do there, because on a literal result element it belongs to the result vocabulary and means whatever that vocabulary says it means; an SVG or XHTML root may well carry its own. The prefixed attribute is also the only thing that makes such a document a stylesheet rather than ordinary XML, so leaving it off does not produce a stylesheet with a missing version — it produces something a processor will not recognise as a stylesheet at all.

A third arrangement carries nothing on its root at all. An embedded stylesheet is a document of data with an xsl:stylesheet inside it, picked out by an xml-stylesheet instruction pointing at that element's id. The version belongs to the embedded element, which declares it as any other stylesheet root does, and the outer root is data — adding xsl:version to it would claim the whole document is a transformation, which is the one thing it is not.

Incorrect:

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
    <!-- stylesheet logic -->
</xsl:stylesheet>

Correct:

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="1.0">
    <!-- stylesheet logic -->
</xsl:stylesheet>

Incorrect:

<html xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
    <body><xsl:value-of select="/page/title"/></body>
</html>

Correct:

<html xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xsl:version="2.0">
    <body><xsl:value-of select="/page/title"/></body>
</html>

Incorrect:

<?xml-stylesheet type="text/xsl" href="#style"?>
<page>
    <xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" id="style">
        <xsl:template match="/"><xsl:value-of select="/page/title"/></xsl:template>
    </xsl:stylesheet>
    <title>Report</title>
</page>

Correct:

<?xml-stylesheet type="text/xsl" href="#style"?>
<page>
    <xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" id="style" version="1.0">
        <xsl:template match="/"><xsl:value-of select="/page/title"/></xsl:template>
    </xsl:stylesheet>
    <title>Report</title>
</page>