title: What do square brackets in class names mean?
body: <p>I saw <a href="http://www.position-absolute.com/articles/jquery-form-validator-because-form-validation-is-a-mess/" rel="noreferrer">here</a> square brackets that are used in class names:</p>
  
  <pre><code>&lt;input class="validate[required,custom[onlyLetter],length[0,100]]" name="firstname" type="text" /&gt;
  </code></pre>
  
  <p>What do these square brackets mean?</p>