While reading https://www.w3.org/XML/Group/qtspecs/specifications/xslt-30/html/Overview-diff.html#stream-examples I noticed that for the example "Using xsl:source-document with xsl:for-each-group" the input and output don't match because the input sample has "chapter" elements with "title" child elements (e.g.

 <chapter part="1">
    <title>The first chapter of book A</title>
    ...
  </chapter>

) while the result document has "title" attributes (e.g. 

    <chapter title="The first chapter of book A" part="1">
      ...
    </chapter>

) although the XSLT does simply use 

         <xsl:for-each-group select="chapter" group-adjacent="data(@part)">
            <part number="{current-grouping-key()}">
               <xsl:copy-of select="current-group()"/>
            </part>
         </xsl:for-each-group>

so it does not modify the "chapter" elements. The document https://www.w3.org/XML/Group/qtspecs/specifications/xslt-30/html/#stream-examples does not have this problem, so somehow the example in the diff.html is different from the one in the spec document.