title: how to append a css class to an element by javascript?
body: <p>Suppose a HTML element's <code>id</code> is known, so the element can be refereced using:</p>
  
  <pre><code>document.getElementById(element_id);
  </code></pre>
  
  <p>Does a native Javascript function exist that can be used to append a CSS class to that element?</p>