title: How is the margin-top percentage calculated?
body: <p>I know this should be straightforward, but can anybody tell me why the child boxes in the following overflow their parent's container when a <code>margin-top: 50%</code> is applied to the child. How is the margin-top percentage calculated?</p>
  
  <p><div class="snippet" data-lang="js" data-hide="false">
  <div class="snippet-code">
  <pre class="snippet-code-css lang-css prettyprint-override"><code>.container {  
    background: lightblue; 
    padding: 10px; 
    height: 200px;
  }
  
  p { 
    display: block; 
    border:1px solid red;
    margin-top:50%;
  }</code></pre>
  <pre class="snippet-code-html lang-html prettyprint-override"><code>&lt;div class="container"&gt;
  &lt;p&gt; Some Cool content&lt;/p&gt;
   
  &lt;/div&gt;</code></pre>
  </div>
  </div>
  </p>
  
  <p>Shouldn't the child element be placed 50% from 200px (set height on parent), i.e 100px from top?</p>