title: <iframe> not displaying pages from the same site, but displaying pages from another site
body: <p>I am deploying an <code>&lt;iframe&gt;</code> on a page on <strong>Site A</strong> as a quick and simple way to display a fragment from a complex page elsewhere on <strong>Site A</strong>:</p>
  <pre><code>&lt;iframe src=&quot;http://site-a.com/complex-page/&quot;&gt;&lt;/iframe&gt;
  </code></pre>
  <p>Nothing displays in the <code>&lt;iframe&gt;</code> on <strong>Site A</strong>.</p>
  <h1>Troubleshooting:</h1>
  <ol>
  <li>I checked <code>http://site-a.com/complex-page/</code> in the browser and the page displays normally.</li>
  <li>I tested the <code>&lt;iframe&gt;</code> by using <code>http://site-b.com/complex-page/</code> as the source and the <code>&lt;iframe&gt;</code> displays the external page on (external) <strong>Site B</strong>, as expected.</li>
  <li>I tested the <code>&lt;iframe&gt;</code> by using <code>http://site-a.com/complex-page-2/</code> as the source and, once again, nothing displays in the <code>&lt;iframe&gt;</code>.</li>
  </ol>
  <p>So... this is clearly something to do with <strong>Site A</strong>, if the <code>&lt;iframe&gt;</code> on <strong>Site A</strong> is not displaying any of several pages on <strong>Site A</strong>, <em>but it is showing any of several pages</em> on <strong>Site B</strong>.</p>
  <p>This clearly isn't a <em>same-origin</em> issue - <strong>Site A</strong> <em>is</em> the same origin as <strong>Site A</strong>.</p>
  <p>So what is preventing the  displaying of the external page on <strong>Site A</strong>?</p>
  <p><strong>Additionally:</strong></p>
  <p>If I <code>Right Click &gt; This Frame &gt; View Frame Source</code> then I <em>can</em> see the HTML Source of the document loaded by the <code>&lt;iframe&gt;</code>... but whatever content or styles I add to the document, nothing displays.</p>
  <p>That suggests the <code>&lt;iframe&gt;</code> is successfully accessing the page but then (for some reason) failing to display it.</p>
  <hr />
  <p><strong>Supporting information:</strong> <em>The site is built with PHP server-side and with HTML5, CSS3 and Javascript client-side.</em></p>