title: simple iframe is not displayed on github pages
body: <p>I regularly embed <code>iframes</code> in my github.io pages but this one fails - I can't see why.</p>
  
  <p>The <code>iframe</code>:</p>
  
  <pre><code>&lt;iframe src="http://www.javarepl.com/embed.html" style="width: 500px; height: 130px; border: 0px"&gt;&lt;/iframe&gt;
  </code></pre>
  
  <p>Embedded at several places in this page - <strong>and not showing up</strong>:</p>
  
  <p><a href="https://seinecle.github.io/codapps/generated-html/essential-notions-of-code.html" rel="noreferrer">https://seinecle.github.io/codapps/generated-html/essential-notions-of-code.html</a></p>
  
  <p>(note: when loading the html page locally, the iframe does show up).</p>