title: CSS negative margin incorrect when using box-sizing: border-box
body: <p>When setting a negative margin to pull a child element to the boundary of the parent element which has padding, the distance moved isn't correct. </p>
  
  <p>Here's a test case:</p>
  
  <pre><code>&lt;div class='full'&gt;
    full
    &lt;div class='child'&gt;
      child
    &lt;/div&gt;
  &lt;/div&gt;
  
  * {
    box-sizing: border-box;
  }
  
  .full {
    width:80%;
    padding-left: 3%;
    background: #000;
    color: #fff;
  }
  .child {
    margin-left: -3.75%;  /* 3 / 80 * 100 */
    padding-left: 3.75%;
    background: #ddd;
    color: #000;
  }
  </code></pre>
  
  <p><a href="http://codepen.io/anon/pen/fuaxE" rel="nofollow">http://codepen.io/anon/pen/fuaxE</a></p>
  
  <p>If I remove the box-sizing the margin is correct, but with border-box set the negative margin is correct - see the line down the left side of the child element. </p>
  
  <p>Am I missing something obvious?</p>