title: How does the blob: protocol work as <video> source?
body: <p>A friend of mine was trying to download a Twitter embedded video and found something like this in the HTML code:</p>
  
  <pre><code>&lt;video preload="auto" data-id="content" data-type="content" src="blob:https%3A//twitter.com/7897de6d-6eed-4905-9ed2-00ea3d2c99d5" class="visible paused" style="width: 100%; height: 100%;"&gt;&lt;/video&gt;
  </code></pre>
  
  <p>I'm as puzzled as he was when I tried to find out the <em>real</em> source of the video stream (either by inspecting the browser network console and hitting a proxy). As the video played, the bytes seem to come out of nowhere.</p>
  
  <p>How does the browser understand the blob "protocol"?</p>