title: Why does Twitter Bootstrap require multiple classes for buttons?
body: <p>To make a primary button using <a href="http://twitter.github.com/bootstrap/base-css.html#buttons" rel="nofollow">Twitter's Bootstrap library</a>, you need to use two CSS classes like so: <code>class="btn btn-primary"</code>.</p>
  
  <p>Why did they design the API this way, when they could have made btn-primary include all of the CSS that btn includes? Is it purely to save on code duplication and therefore file size or is there a more complex reason?</p>