Excluding the use of xsl:result-document, is the result of an XSLT
transformation always a document-node() ?

The spec talks about result trees, but couldn't figure out if the tree
is always rooted at a document-node() or if a tree rooted at an
element() is allowed.

<xsl:template match="/">
  <xsl:copy-of select="*" />
  <xsl:copy-of select="*" />
</xsl:template>