title: Add class in html tag with jquery
body: <p>I got this html code:</p>
  
  <pre><code>&lt;form id = "xxx"&gt;
     &lt;ul&gt;
       &lt;li class = "req" &gt; &lt;/li&gt;
       &lt;li class = "req" &gt; &lt;/li&gt;
       &lt;li class = "req" &gt; &lt;/li&gt;
       &lt;li &gt; &lt;/li&gt;    ==================&gt; HOW CAN I ADD A CLASS="req" to this li with jquery
       &lt;li class = "req" &gt; &lt;/li&gt;
       ....
      &lt;li class = "req" &gt; &lt;/li&gt;
    &lt;/ul&gt;
  &lt;/form&gt;
  </code></pre>
  
  <p>So I rewrite the question: There is one li tag that has not ( class= "req"), how can i add that with jquery?
  Thank you</p>