title: What's the difference between a block-level box and a principal block-level box?
body: <p>To cite the <a href="http://www.w3.org/TR/CSS21/visuren.html" rel="noreferrer">spec</a>:</p>
  
  <blockquote>
    <p>Block-level boxes are boxes that participate in a block formatting
    context. Each block-level element generates a principal block-level
    box that contains descendant boxes and generated content and is also
    the box involved in any positioning scheme. Some block-level elements
    may generate additional boxes in addition to the principal box:
    'list-item' elements. These additional boxes are placed with respect
    to the principal box.</p>
  </blockquote>
  
  <p>Are they essentially the same thing?</p>