I'm trying to display the title of the feed in verdana 12px, so I've scripted this in the .xsl file :

<a href="{link}" target="_new">
<font size="12px" font-family="verdana">
<xsl:value-of select="substring(title,1,60)"/>
</font>
</a>

or this with no more luck :

<font size="12px" font-family="verdana">
<a href="{link}" target="_new">
<xsl:value-of select="substring(title,1,60)"/>
</a>
</font>

The display is changed, but not at all as expected (letters are becoming very large like 64px or so). What is the right way to code css-like formatting elements in an xsl file ?

Of course ! I had my head so deap in those xsl , xls-fo , xslt specs,  that I forgot my HTML basics...

About creating a style class, I agree in general, but in this case what I (we ?) want is create a set of .xsl files that we could reuse in most DNN sites. So I prefer an .xsl file that will be standalone and not require any extra settings in the style sheet. Furthermore, our content is basically a title and a description, so having to depend on 4 or 5 different places to set the the display of such a simple content is somewhat opposite to DNN's philosophy of beeing easy, even if it's good programming practice.

In fact, title and description's lenght, size and color should probably be module settings in a future version. That would be the most simple way for non-programmers.

Finally, I've changed RSSDisplayHtml.xsl so that only the first 60 characters of the title are displayed, and if it had to be truncated, I add "...". 

I did that because I noticed one problem with the concept of 'channels'  (a great thing !) is that rss sources can have very different sizes for their titles and description, so you may want to uniformize all this. Here is the portion of the .xsl :

<a href="{link}" target="_new" style="font-size:12px;font-family:verdana;">
<xsl:value-of select="substring(title,1,60)"/>
<xsl:if test="string-length(title)&gt;60">
     <xsl:text>...</xsl:text>
</xsl:if>
</a>