::before和::after 是用来给元素添加额外内容的,因为只存在于作用元素内容的前后
在css中，::before和::after 是一个伪类元素，代表生成的内容元素，表示相应元素的可抽象样式的第一个子元素，即：所选元素的第一个子元素。
伪元素可用于插入几乎任何类型的内容，包括字符,图片，字符串
.element :: before {
content：url（path / to / image.png）; / *图像，例如，图标* /
}
.element :: before {
content：“注意：” ; / *一个字符串* /
}
.element :: before {
content：“\201C” ; / *也算作一个字符串。转义Unicode会将其渲染为字符* /
}
说明：
1、伪类元素必须要配合content属性一起使用
2、伪类元素是css渲染层加入的，不能通过js来操作
3、伪类对象特效通常通过：hover伪类样式来激活
案例参考
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>before 和 after</title>
<style>
.wrapper * {
box-sizing: border-box;
}
.wrapper>div {
height: 250px;
width: 250px;
border: 1px solid #ccc;
background: #000;
margin: 10px;
float: left;
color: #D9D9D9;
padding: 30px;
text-align: center;
}
/*基础用法1*/
.base1:before {
content: "我是::before";
color: #FB0D0D;
}
.base1:after {
content: "我是::after";
color: #f70;
}
/*基础用法2*/
.base2:before {
content: '\ABCD\ABCD\ABCD\ABCD';
white-space: pre;
color: #FB0D0D;
}
.base2:after {
content: '\609\609\609\609\609';
white-space: pre;
color: #f70;
}
/*::before , ::after添加背景图*/
.base3:before {
content: url(icon-plus.png);
}
.base3:after {
content: url(icon-plus.png);
}
/*取自定义属性*/
.base4:before {
content: attr(title);
color: #E8E3AA;
}
.base4:after {
content: attr(data-test);
color: #D8CF23;
}
/*小试身手合集*/
.base5,
.base6,
.base7,
.base8 {
position: relative;
}
/*小试身手1*/
.base5:before {
content: "";
display: block;
position: absolute;
top: 0;
left: 0;
height: 50px;
width: 50px;
border-top: 5px solid #f70;
border-left: 5px solid #f70;
}
.base5:after {
content: "";
display: block;
position: absolute;
right: 0;
bottom: 0;
height: 50px;
width: 50px;
border-right: 5px solid #f70;
border-bottom: 5px solid #f70;
}
/*小试身手2*/
.base6:before {
content: "";
display: block;
position: absolute;
left: 0;
top: 0;
width: 100%;
height: 100%;
background: -webkit-linear-gradient(45deg, rgba(75, 65, 45, .3), rgba(123, 456, 789, .3), rgba(854, 183, 729, .3));
background: linear-gradient(45deg, rgba(75, 65, 45, .3), rgba(123, 456, 789, .3), rgba(854, 183, 729, .3));
z-index: 1;
}
.base6:after {
content: "";
display: block;
position: absolute;
bottom: 0;
right: 0;
width: 100%;
height: 100%;
background: -webkit-linear-gradient(-45deg, rgba(50, 125, 55, .7), rgba(55, 3, 45, .5), rgba(99, 12, 3, .8));
background: linear-gradient(-45deg, rgba(50, 125, 55, .7), rgba(55, 3, 45, .5), rgba(99, 12, 3, .8));
z-index: 1;
}
/*小试身手3*/
.base7:before {
content: "";
display: block;
position: absolute;
width: 50px;
height: 50px;
-webkit-animation: circle 2s ease-in-out infinite;
-moz-animation: circle 2s ease-in-out infinite;
-ms-animation: circle 2s ease-in-out infinite;
-o-animation: circle 2s ease-in-out infinite;
animation: circle 2s ease-in-out infinite;
background: #C3172C;
}
.base7:after {
content: "";
display: block;
position: absolute;
content: "";
background: #14965E;
display: block;
position: absolute;
width: 50px;
height: 50px;
-webkit-animation: circle 2s ease-in-out infinite;
-moz-animation: circle 2s ease-in-out infinite;
-ms-animation: circle 2s ease-in-out infinite;
-o-animation: circle 2s ease-in-out infinite;
animation: circle 2s ease-in-out infinite;
}
/*小试身手4*/
.base8:before {
content: "1";
display: block;
position: absolute;
height: 100%;
width: 10px;
background: #6F0ECF;
left: 0;
top: 0;
margin-left: -10px;
}
.base8:hover:before {
background: #9F81DE;
-webkit-transform: rotate(-90deg) translate(-30%, 30%);
transform: rotate(-90deg) translate(-30%, 30%);
-webkit-transition: all 2s ease-in;
transition: all 2s ease-in;
}
.base8:after {
content: ".";
display: block;
position: absolute;
height: 100%;
width: 10px;
background: #6F0ECF;
right: 0;
bottom: 0;
margin-right: -10px;
}
.base8:hover:after {
background: #9F81DE;
-webkit-transform: rotate(-90deg) translate(-30%, 30%);
transform: rotate(-90deg) translate(-30%, 30%);
-webkit-transition: all 2s ease-in;
transition: all 2s ease-in;
}
@-webkit-keyframes circle {
from {
border-radius: 0%;
top: 0;
}
35% {
background: #2B2FDC;
left: 30%;
top: 50%;
}
75% {
background: #AB9E9E;
right: 0;
bottom: 20%;
}
to {
border-radius: 100%;
top: 250px;
left: 15%;
bottom: 50%;
}
}
@-moz-keyframes circle {
from {
border-radius: 0%;
top: 0;
}
35% {
background: #2B2FDC;
left: 30%;
top: 50%;
}
75% {
background: #AB9E9E;
right: 0;
bottom: 20%;
}
to {
border-radius: 100%;
top: 250px;
left: 15%;
bottom: 50%;
}
}
@keyframes circle {
from {
border-radius: 0%;
top: 0;
}
35% {
background: #2B2FDC;
left: 30%;
top: 50%;
}
75% {
background: #AB9E9E;
right: 0;
bottom: 20%;
}
to {
border-radius: 100%;
top: 250px;
left: 15%;
bottom: 50%;
}
}
</style>
</head>
<body>
<div class="wrapper">
<div class="base1">添加文字</div>
</div>
<div class="wrapper">
<div class="base2">添加unicode文字,图标的需要特殊字体库[font-face引入webfont,内部的unicode编码]</div>
</div>
<div class="wrapper">
<div class="base3">添加背景图,但是不如用background好控制</div>
</div>
<div class="wrapper">
<div class="base4" title="呵呵哒,萌萌哒" data-test="我是HTML5自定义属性">取自定义属性,取值不带双引号!!</div>
</div>
<div class="wrapper">
<div class="base5">小试身手1:框框</div>
</div>
<div class="wrapper">
<div class="base6">小试身手2:渐变</div>
</div>
<div class="wrapper">
<div class="base7">小试身手3:变形</div>
</div>
<div class="wrapper">
<div class="base8">小试身手4:追加</div>
</div>
</body>
</html>
::before和::after的与伪类的结合使用是有先后顺序的;
.class:hover::before{} /*是有效的*/
.class::before:hover{} /*是无效的*/
内容居中标题案例
.title-line { text-align: center; margin: 2rem 0; position: relative; }
.title-line h2 { font-size: 1.8rem; width: 10rem; height: 2rem; line-height: 2rem;margin: 0 auto; display: block; }
.title-line h2::before { display: inline-block; width: 5rem; height: 0.1rem; background: #eee; position: absolute; left: calc(50% - 10rem); content: ''; top: 1rem; }
.title-line h2::after { display: inline-block; width: 5rem; height:0.1rem; background: #eee; position: absolute;right: calc(50% - 10rem); content: ''; top: 1rem; }