css——层叠样式表,简单理解就是用来装饰网页的样子，比如颜色，大小，位置等等。总之，你现在能看到美轮美奂的网页就是css的功劳。
今天介绍一下css中伪元素before、after的妙用小技巧。
首先纠正下写法：
伪类用一个冒号表示 :hover
伪元素则使用两个冒号表示 ::before
小例子一 导航鼠标悬停特效
<html>
<style>
ul {
padding: 0;
margin: 0;
font-size: 12px;
}
li {
font-size: 20px;
padding: 5px 15px 5px 15px;
display: inline-block;
position: relative;
cursor: pointer;
transition: all 0.5s;
}
/*默认设置left=100%表示在最右边*/
li::before {
content: '';
display: block;
position: absolute;
width: 0%;
bottom: 0;
left: 100%;
border-bottom: 2px solid #999;
/*all 0.5秒 表示所有类型的过渡动画都在0.5秒内完成*/
transition: all 0.5s;
}
/*鼠标hover上去的时候width从0变化至100*/
li:hover::before {
left:0;
width:100%;
}
/*表示设置位于hover之后的li的before的left为0。实现向右移动的时候下面的线条从左往右出现*/
li:hover ~ li::before {
left:0;
}
</style>
<ul>
<li>第一栏</li>
<li>第二栏</li>
<li>第三栏</li>
<li>第四栏</li>
<li>第五栏</li>
</ul>
</html>
小例子二：手机滑盖效果
<style type="text/css">
div#phone li:before{
content: url(./img/wangwang.jpg);
position: relative;
display: block;
transition: transform 1s;
}
div#phone li:after{
content: url(./img/bujibuji.jpg);
position: relative;
top: -750px;
display: block;
transition: transform 1s;
}
div#phone li:hover:before{
transform: translate(-40%,0);
}
div#phone li:hover:after{
transform: translate(40%,0);
}
li{
list-style-type: none;
width: 500px;
margin: 20px auto;
}
</style>
<div id="phone">
<li></li>
</div>
小例子三 翻转效果
<html>
<style>
*,
*::after,
*::before {
-webkit-box-sizing: border-box;
-moz-box-sizing: border-box;
box-sizing: border-box;
}
nav a {
position: relative;
display: inline-block;
margin: 15px 25px;
outline: none;
color: #fff;
text-decoration: none;
text-transform: uppercase;
letter-spacing: 1px;
font-weight: 400;
text-shadow: 0 0 1px rgba(255,255,255,0.3);
font-size: 1.35em;
}
nav a:hover,
nav a:focus {
outline: none;
}
.cl-effect-2 a {
line-height: 44px;
-webkit-perspective: 1000px;
-moz-perspective: 1000px;
perspective: 1000px;
}
.cl-effect-2 a span {
position: relative;
display: inline-block;
padding: 0 14px;
background: #2195de;
-webkit-transition: -webkit-transform 0.3s;
-moz-transition: -moz-transform 0.3s;
transition: transform 0.3s;
-webkit-transform-origin: 50% 0;
-moz-transform-origin: 50% 0;
transform-origin: 50% 0;
-webkit-transform-style: preserve-3d;
-moz-transform-style: preserve-3d;
transform-style: preserve-3d;
text-align: center;
}
.cl-effect-2 a span::before {
position: absolute;
top: 100%;
left: 0;
width: 100%;
height: 100%;
background: #0965a0;
content: attr(data-hover);
-webkit-transition: background 0.3s;
-moz-transition: background 0.3s;
transition: background 0.3s;
-webkit-transform: rotateX(-90deg);
-moz-transform: rotateX(-90deg);
transform: rotateX(-90deg);
-webkit-transform-origin: 50% 0;
-moz-transform-origin: 50% 0;
transform-origin: 50% 0;
text-align: center;
}
.cl-effect-2 a:hover span,
.cl-effect-2 a:focus span {
-webkit-transform: rotateX(90deg) translateY(-22px);
-moz-transform: rotateX(90deg) translateY(-22px);
transform: rotateX(90deg) translateY(-22px);
}
.cl-effect-2 a:hover span::before,
.cl-effect-2 a:focus span::before {
background: #28a2ee;
}
.cl-effect-19 a {
line-height: 2em;
-webkit-perspective: 800px;
-moz-perspective: 800px;
perspective: 800px;
}
.cl-effect-19 a span {
position: relative;
display: inline-block;
width: 100%;
padding: 0 14px;
background: #e35041;
-webkit-transition: -webkit-transform 0.4s, background 0.4s;
-moz-transition: -moz-transform 0.4s, background 0.4s;
transition: transform 0.4s, background 0.4s;
-webkit-transform-style: preserve-3d;
-moz-transform-style: preserve-3d;
transform-style: preserve-3d;
-webkit-transform-origin: 50% 50% -60px;
-moz-transform-origin: 50% 50% -60px;
transform-origin: 50% 50% -60px;
text-align: center;
}
.cl-effect-19 a span::before {
position: absolute;
top: 0;
left: 100%;
width: 100%;
height: 100%;
background: #b53a2d;
content: attr(data-hover);
-webkit-transition: background 0.4s;
-moz-transition: background 0.4s;
transition: background 0.4s;
-webkit-transform: rotateY(90deg);
-moz-transform: rotateY(90deg);
transform: rotateY(90deg);
-webkit-transform-origin: 0 50%;
-moz-transform-origin: 0 50%;
transform-origin: 0 50%;
pointer-events: none;
text-align: center;
}
.cl-effect-19 a:hover span,
.cl-effect-19 a:focus span {
background: #b53a2d;
-webkit-transform: rotateY(-90deg);
-moz-transform: rotateY(-90deg);
transform: rotateY(-90deg);
}
.cl-effect-19 a:hover span::before,
.cl-effect-19 a:focus span::before {
background: #ef5e50;
}
</style>
<section class="color-4">
<nav class="cl-effect-2" id="cl-effect-2">
<a href="#cl-effect-2"><span data-hover="上下翻转">上下翻转</span></a>
</nav>
<nav class="cl-effect-19" id="cl-effect-19">
<a href="#cl-effect-19"><span data-hover="左右翻转">左右翻转</span></a>
</nav>
</section>
</html>