My RSS feeds include a short plain text description of each post in the <description> element within each <item>.

These aren't rendered by the current pretty-feed-v3.xsl file.

I've added something like this, between the item title and Published date:

<p class="mb-0">
  <xsl:choose>
    <xsl:when test="string-length(description) &gt; 200">
      <xsl:value-of select="substring(description, 0, 200)" />…
    </xsl:when>
    <xsl:otherwise>
      <xsl:value-of select="description" />
    </xsl:otherwise>
  </xsl:choose>
</p>

This displays the entire <description> if it's 200 characters or fewer, otherwise displays the first 200 characters followed by an ellipsis. (200 is just arbitrary.) e.g.:

But I think some people have the entire HTML blog post within <description>, between <![CDATA[ and ]]>. This might be problematic if it's truncated mid-tag, or between opening and closing tags? Or maybe the HTML itself would be visible?

AFAIK there isn't a way to detect if the description starts with ![CDATA[ or not, so I'm not sure if this idea can be reliably generalised.

But, given I know my feeds only have a brief plaintext description, I've gone with the above.