title: CSS '1px solid #000;' producing double border
body: <p>I'm puzzled. I swear I have used this CSS before and it produced a 1px border without problems.</p>
  
  <p>The CSS is:</p>
  
  <pre><code>#menu-primary-menu-1 {
      border: 1px solid #000;
      padding: 1em;   
  }
  </code></pre>
  
  <p>It is being applied around the top navigation of <a href="http://test.doig.com.au/meyer/" rel="nofollow noreferrer">this page</a>.</p>
  
  <p>However, it is producing a double-border.</p>
  
  <p><a href="https://i.stack.imgur.com/RgC4i.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/RgC4i.png" alt="enter image description here"></a></p>
  
  <p>I can see no CSS being applied here which is producing this double border. Can you?</p>