disable-output-escaping="yes"

<xsl:choose>
    <xsl:when test="string-length(fields/field_article/data) > 900">
      <xsl:value-of select="substring(fields/field_article/data,0, 900)" disable-output-escaping="yes"/>...
    </xsl:when>
    <xsl:otherwise>
    <xsl:value-of select="fields/field_article/data" disable-output-escaping="yes"/>
    </xsl:otherwise>
  </xsl:choose>