SOURCE

var contentVar = ['task1', 'task2', 'task3', 'task4', 'task5', 'task6', 'task7', 'task8' ];
var len = $('.content').length;
for (var i = 0; i < len; i++) {
    $('.content').eq(i).addClass(contentVar[i]);
}

<script type="text/javascript" src="http://n.sinaimg.cn/sifa/video/js/jquery-1.11.0.min.js"></script>
<p class="content">11111</p>
<p class="content">222222</p>
<div class="content">3</div>
<p><span class="content">4</span></p>
<ul>
  <li class="content">5</li>
  <li>6</li>
  <li>7</li>
</ul>
<p class="content">8</p>

.content {
   font-style:italic;
}
.task1,.task2,.task3,.task4,.task5,.task6,.task7,.task8 {
  background-color: #000;
  color: #fff;
}