When first Node add a style in XSL

How do I put the proper test in for the first Node I encounter, add a
top border:?

    <xsl:template match="category">
        <xsl:choose>
            <xsl:when test="first::category">
                <xsl:element name="h3">
                    <xsl:attribute name="style">border-top:
1px</xsl:attribute>
                    <xsl:value-of select="@display_name"/>
                </xsl:element>
            </xsl:when>
            <xsl:otherwise>
                <xsl:element name="h3">
                    <xsl:value-of select="@display_name"/>
                </xsl:element>
            </xsl:otherwise>
        </xsl:choose>