:after和:before的作用及使用方法

<p>你好</p>

p:before{
   content: ‘Hello‘;
   color: red;
}
p:after{
    content: ‘Tom‘;
    color: red;
}

<div class="demo">这是一个测试</div>

.demo:after{
    content: ‘‘;
    display: inline-block;
    width: 0;
    height: 0;
    border: 8px solid transparent;
    border-left: 8px solid #AFABAB;
    position: relative;
    top: 2px;
    left: 10px;
}