We should note that the general issue of whether the xsl:output can
affect the construction of the result tree goes beyond the specific
case cited here. For instance, if it is OK for xsl:output to do this,
then one might imagine that <xsl:output indent="yes"/> could lead to
the insertion of additional text nodes in the result tree, though to
my knowledge no XSLT processor yet does that.
It certainly can't put additional text nodes in the result tree. But it
could put them in a DOM or SAX representation of the result tree. Since the
spec doesn't define the mapping of the result tree to DOM or SAX, vendors
can do what they like.
Michael Kay