This *should be* obvious, but sometimes the obvious isn't always to this
observer.  :)

   <xsl:call-template name="detail">
      <xsl:with-param name="category">markup</xsl:with-param>
   </xsl:call-template>

...

   <xsl:template name="detail">
     <xsl:param name="category" />
       <xsl:for-each select='/config/option[@class="$category"]'>
         ...
       </xsl:for-each>
   </xsl:template>

The for-each selects *nothing*, but if I change "$category" to the
hard-coded string "markup", it works fine!

The `$category` parameter does come through, or so

<xsl:value-of select='$category' />

tells me.  So why does does a hard-coded category work, while "$category"
fails?

/Jelks

You don't need quotes around it if its a variable, compare:
x = a + b; //a and b are variables
x = "a" + "b"; // giving "ab" as a and b are strings