CSS3 : 伪元素 ::before 和 ::after的用法介绍
前言
单冒号写法兼容性比较强[可支持解析的浏览器较多],双冒号的写法只支持较新的主流浏览器;
W3C的解释用一句话概括：单冒号(:)用于CSS3伪类，双冒号(::)用于CSS3伪元素。
基础理论
::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::before和::after内部的content支持以下三种特性!- 字符串[“可以是符号什么的也可以是单纯的字符” ,支持unicode编码!!]
- 属性[
attr(), 可以获取标签上的元素属性,比如data-*的自定义属性,title,alt这些] - 引用媒体文件[url ,可以链接图片作为背景图什么的]
- 计数器[
counter()]
灵活运用这两个可以做好多东东,比如清除浮动,各种额外的视觉效果[阴影,跳转等]
效果
代码
<html lang="en">
<head>
<meta charset="UTF-8">
<title>before 和 aftertitle>
<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>
注意点
::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*/