Avoid collapsed tags

["Hvid Hat"]

I'm using <xsl:copy-of select="content" /> on the following XML:

<?xml version="1.0"?>
<content>
  <p></p>
  <p>This is a paragraph</p>
  <p></p>
  <p>This is another paragraph</p>
</content>

The empty paragraphs, <p></p>, become <p/> when outputted to the browser. 

How can I prevent this? I want to add an empty comment (or something else?)
so the paragraph tags aren't collapsed, eg. <p><!-- --></p>