Just replace 'xsl:value-of' elements with 'xsl:apply-templates' elements in the templates:

<xsl:template match="bold">
    <xsl:element name="strong">
      <xsl:apply-templates/>
    </xsl:element>
</xsl:template>
<xsl:template match="emph">
    <xsl:element name="em">
      <xsl:apply-templates/>
    </xsl:element>
</xsl:template>

Tank you very much for your suggestions Joris; now it works :-)
bye