这两个伪类下特有的属性 content ，用于在 CSS 渲染中向元素逻辑上的头部或尾部添加内容。注意这些添加不会改变文档内容，不会出现在 DOM 中，不可复制，仅仅是在 CSS 渲染层加入
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style type="text/css">
a {
position: relative;
display: inline-block;
outline: none;
text-decoration: none;
color: #000;
font-size: 32px;
padding: 5px 10px;
}
a:hover::before, a:hover::after { position: absolute; }
a:hover::before { content: "\5B"; left: -20px; }
a:hover::after { content: "\5D"; right: -20px; }
.triangle{
width: 0;
height: 0;
border-left:50px solid red;
border-bottom:50px solid blue;
border-top:50px solid black;
border-right:50px solid purple
}
p:before{
content: "H" /*:before和:after必带技能，重要性为满5颗星*/
}
p:after{
content: "d" /*:before和:after必带技能，重要性为满5颗星*/
}
.test-div{
position: relative; /*日常相对定位*/
width:150px;
height: 36px;
border:1px solid black;
border-radius:5px;
background: rgba(245,245,245,1)
}
.test-div:before,.test-div:after{
content: ""; /*:before和:after必带技能，重要性为满5颗星*/
display: block;
position: absolute; /*日常绝对定位*/
top:5px;
width: 0;
height: 0;
border:6px solid transparent;
}
.test-div:before{
left:-11px;
border-right-color: rgba(245,245,245,1);
z-index:1
}
.test-div:after{
left:-12px;
border-right-color: rgba(0,0,0,1);
z-index: 0
}
</style>
</head>
<body>
<a href="">按钮一</a>
<a href="">按钮二</a>
<div class="triangle"></div>
<p>ello Worl</p>
<div class="test-div">ddd</div>
</body>
</html>结果如下:
