title: HTML window inside window
body: <p>I've been searching around the web for a very long time on how to get a window inside a window, not just a popup window.</p>
  
  <p>For example, clicking "RyanM" on this page:<br>
  <img src="https://i.stack.imgur.com/qUq7M.png" alt=""></p>
  
  <p>does this:<br>
  <img src="https://i.stack.imgur.com/mhCY6.png" alt=""></p>
  
  <p>But how do you do it, and is it possible to do with only HTML and CSS?</p>