title: Set multiple values for attribute
body: <p>It's easy to set attribute for a given element:</p>
  
  <p><code>el.setAttribute('rel', 'nofollow');</code></p>
  
  <p>But for some attributes, such as <code>rel</code> in the example, multiple values are allowed. In case of <code>rel</code>, its values should be <a href="http://www.w3.org/TR/html401/struct/links.html#adef-rel" rel="nofollow">separated by space</a>. Therefore, I cannot just <code>setAttribute</code>, because that would replace the existing value. What's the right way to <em>append</em> value to an existing attribute, so that it'll contain the both values, such as <code>rel="external nofollow"</code>? Is there anything better than getting the existing attribute and <code>+=</code> the new value?</p>
  
  <p>Note: Ideally, I don't want to use jQuery, just plain JavaScript.</p>