Figured it out by myself:

<xsl:template match="style">	
	<xsl:copy>	
		<xsl:apply-templates select="@*"/>		
		<xsl:call-template name="stylehead">
			<xsl:with-param name="s" select="."/>          
		</xsl:call-template>		
	</xsl:copy>
</xsl:template>

<xsl:template name="stylehead">
	<xsl:param name="s"/>	
	<xsl:choose>		
		<xsl:when test="contains($s, 'font-size:')">
			<!--Keep text up to-->
			<xsl:value-of select="substring-before($s,'font-size:')"/>
			<xsl:call-template name="styletail">
				<xsl:with-param name="s" select="substring-after($s, 'font-size:')"/>
			</xsl:call-template>			
		</xsl:when>
		<xsl:otherwise>
			<xsl:value-of select="$s"/>
		</xsl:otherwise>
	</xsl:choose>
</xsl:template>

<xsl:template name="styletail">
	<xsl:param name="s"/>	
	<xsl:if test="string-length($s)>0">
		<!-- keep value, add unit-->
		<xsl:value-of select="concat('font-size:', substring-before($s,';'), 'px;')"/>
		<xsl:call-template name="stylehead">
			<xsl:with-param name="s" select="substring-after($s, ';')"/>
		</xsl:call-template>
	</xsl:if>
</xsl:template>

Ralph Wiggum wrote:
> I need to add 'px' to font-size values like those in this xml:
> <style>   .class1{
>      font-family:Arial;
>      font-size:25;
>   }
>   .class2{      font-family:Arial Bold;
>      font-size:20;
>   }
> </style>
> 
> So it should be for example font-size:25px;
> 
> Can someone please give some tips on how to do this with xsl? I have to 
> use MSXML4.