title: Why doesn't the <span> tag behave the same as a <div> tag?
body: <p>Why doesn't the circle appear using a span tag, but does with a div tag? 
  <a href="http://jsfiddle.net/hqb2e9n0/" rel="nofollow">FIddle</a></p>
  
  <pre><code>&lt;span class="circle red"&gt;&lt;/span&gt;
  &lt;div class="circle red"&gt;&lt;/div&gt;
  
  .circle {
      width: 15px;
      height: 15px;
      border-radius: 50%;
  }
  .red {
      background: red;
  }
  </code></pre>