So, you want to add some spaces to your XSLT, shouldn't be a big deal right? Well, the first thing I'd try is:
<xsl:text> </xsl:text>
Oh, but that's right, we're generating HTML as the output, so you need to use some &nsbp;
characters to acutally get the spaces to show up:
<xsl:text>, </xsl:text>
Phew, don...oh...that doesn't validate?
reference to undeclared entity 'nsbp'. Line blah...
Hmmm, so what is expected here then? Well it's actually expecting the HTML hexidecimal entity representation of the &nsbp;
character, which happens to be  
(I know that thanks to http://www.fileformat.info/info/unicode/char/a0/index.htm):
<xsl:text>, </xsl:text>
The stylesheet will now validate and can insert spaces where you'd like them to appear.
Neat.