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伪元素, 伪类 例子

http://codepen.io/jplhomer/pen/lgfus

<div class="timeline"></div>
OR
<ul class="timeline"></ul>

.timeline:before {
    content: '';
    position: absolute;
    height: 100%;
    width: 2px;
    background: #333;
    left: 0;
}