Malformed version in stylesheet

The version attribute is what a processor reads to decide which language it is running, and it is declared as an xs:decimal. That is why 2, 2.0 and 2.00 all name version 2.0 and behave alike, and why a value that is not a decimal at all names nothing: 2,0 is a typo for a version, 2e0 is a spelling the type does not have, and an empty version says as much as no version would.

A processor is free to answer any of those differently, and the quiet answer is the dangerous one. Where the effective version cannot be read as 1.0, the stylesheet is not in XSLT 1.0 behaviour, so a processor that shrugs and carries on may run xsl:for-each-group, sequence types and every other construct in the file under rules the author never chose — or it may reject the stylesheet outright. Which of the two you get is not something the file decides.

Incorrect:

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="2,0">
  <xsl:template match="/">
    <xsl:value-of select="count(item)"/>
  </xsl:template>
</xsl:stylesheet>

Correct:

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="2.0">
  <xsl:template match="/">
    <xsl:value-of select="count(item)"/>
  </xsl:template>
</xsl:stylesheet>

The same applies wherever a version is declared, not only at the root: XSLT 2.0 lets version sit on any XSLT element and xsl:version on any literal result element, each setting the version of everything beneath it. A template raised to a value that names no version leaves that subtree in the same doubt a whole file would be in.

A version that is a decimal is a different matter, even one no processor has shipped yet. version="4.0" is well formed and says plainly which language the author wants; a 3.0 processor runs it forwards-compatibly, and the only cost is that instructions from a later version go unrecognised. One exception is worth knowing about: version on xsl:output is not the language at all but the version of the output method, so 4.0 there asks for HTML 4.0 and is ordinary. On xsl:result-document the serialization parameter is spelled output-version, precisely so the two do not collide — writing version there declares the language for everything the instruction contains.