Synopsis

    <xsl:attribute-set name="slides.titlepage.subtitle.properties">
      <xsl:attribute name="text-align">center</xsl:attribute>
      <xsl:attribute name="space-after">1em</xsl:attribute>
      <xsl:attribute name="font-family">
	<xsl:value-of select="$slide.title.font.family"></xsl:value-of>
      </xsl:attribute>
    </xsl:attribute-set>