Just now when writing a transform, for temporary diagnostic purposes I
needed to produce messages showing (1) the initial string value of an
input element before template application, and (2) the output string
value after template application. Oddly enough I have never before
wanted to do precisely that task.

A moment's reflection suggested that I could capture the result of
<xsl:apply-templates/> and use it as a variable, thus (given XSLT 2.0):

<xsl:template match="INPUT">
<xsl:variable name="appliedTemplates" as="item()+">
<xsl:apply-templates/>
</xsl:variable>