Before和After用法小结
定义 :before 选择器在被选元素的内容前面插入内容。:after选择器在被选元素的内容后面插入内容。（注：必须包含content 属性）
一、特性：不能左右:empty伪类
此表现说明，空元素内部使用伪元素生成的内容，是不被:empty伪类认可的，选择器依然认为这是个空元素
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>不能左右:empty伪类</title>
<style>
.box{
width: 256px;
height: 90px;
padding: 10px;
background-color: red;
color:#fff;
}
.box:empty{
opacity: .1;
}
.pseudo{
content: "伪元素生成内容";
}
</style>
</head>
<body>
<div class="item">
<div class="title">有内容</div>
<div class="box">有内容</div>
</div>
<div class="item">
<div class="title">无内容</div>
<div class="box"></div>
</div>
<div class="item">
<div class="title">空格也算内容</div>
<div class="box"> </div>
</div>
<div class="item">
<div class="title">伪元素不算内容</div>
<div class="box pseudo"></div>
</div>
</body>
</html>
二、特性：content动态呈现值无法获取
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
<style>
body {
counter-reset: icecream;
}
input:checked {
counter-increment: icecream;
}
.total::after {
content: counter(icecream);
}
</style>
</head>
<body>
<strong>下面中国十大冰淇淋你吃过几个？</strong>
<ul>
<li><input type="checkbox" id="icecream1"><label for="icecream1">哈根达斯</label></li>
<li><input type="checkbox" id="icecream2"><label for="icecream2">和路雪wall‘s</label></li>
<li><input type="checkbox" id="icecream3"><label for="icecream3">八喜冰淇淋</label></li>
<li><input type="checkbox" id="icecream4"><label for="icecream4">DQ冰淇淋</label></li>
<li><input type="checkbox" id="icecream5"><label for="icecream5">蒙牛冰淇淋</label></li>
<li><input type="checkbox" id="icecream6"><label for="icecream6">雀巢冰淇淋</label></li>
<li><input type="checkbox" id="icecream7"><label for="icecream7">伊利冰淇淋</label></li>
<li><input type="checkbox" id="icecream8"><label for="icecream8">乐可可冰淇淋</label></li>
<li><input type="checkbox" id="icecream9"><label for="icecream9">新城市冰淇淋</label></li>
<li><input type="checkbox" id="icecream10"><label for="icecream10">明治MEIJI</label></li>
</ul>
你总共选择了 <strong class="total"></strong> 款冰淇淋！
</body>
</html>
三、用法：清除浮动
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
<style>
.clearfix:before,.clearfix:after { content: "."; display: block; height: 0; visibility: hidden; }
.clearfix:after {clear: both;} .clearfix {zoom: 1;} /* IE < 8 */
.box div{width: 200px;height: 200px;background-color: red;margin-right: 20px;margin-bottom: 20px;float: left; }
.test{clear: both;margin-top: 80px}
</style>
</head>
<body>
<div class="test">测试一：</div>
<div class="box"><div></div><div></div></div>
<div class="box"><div></div><div></div></div>
<!--*********************************************************-->
<div class="test">测试二：</div>
<div class="box clearfix"><div></div><div></div></div>
<div class="box"><div></div><div></div></div>
</body>
</html>
四、用法：做时间轴
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
<style>
ul{
width: 635px;
height: 100px;
padding: 20px;
list-style: none;
background-color: #f5f5f5;
}
li{
text-align: center;
position: relative;
float: left;
padding: 2px 110px 2px 12px;
}
.cusicon{
margin: 0 auto 15px auto;
position: relative;
}
.tip,.time{
position: absolute;
left: -40px;
text-align: center;
width: 140px;
font-size: 14px;
}
.time{
top:75px;
font-size: 12px;
}
.last:before{
width: 0!important;
}
.cusicon:before{
content:"";
width: 100px;
height: 3px;
position: absolute;
top:17px;
right:-115px;
background-color: #b3b3b3;
}
li.active .cusicon{
background: url(sprite.png) -0px -72px no-repeat;
width: 100%;
height: 100%;
_background: none;
_padding-left: 0px;
_margin-left: -0px;
_padding-top: 72px;
_margin-top: -72px;
_filter: progid:DXImageTransform.Microsoft.AlphaImageLoader(sizingMethod=crop,src="sprite.png");
}
li.active .cusicon:before{
background-color: red!important;
}
.cusicon {
height: 36px!important;
width: 36px!important;
}
.cusicon_1 {
background: url(sprite.png) -0px -0px no-repeat;
width: 100%;
height: 100%;
_background: none;
_padding-left: 0px;
_margin-left: -0px;
_padding-top: 0px;
_margin-top: -0px;
_filter: progid:DXImageTransform.Microsoft.AlphaImageLoader(sizingMethod=crop,src="sprite.png");
}
.cusicon_2 {
background: url(sprite.png) -36px -0px no-repeat;
width: 100%;
height: 100%;
_background: none;
_padding-left: 36px;
_margin-left: -36px;
_padding-top: 0px;
_margin-top: -0px;
_filter: progid:DXImageTransform.Microsoft.AlphaImageLoader(sizingMethod=crop,src="sprite.png");
}
.cusicon_3 {
background: url(sprite.png) -0px -36px no-repeat;
width: 100%;
height: 100%;
_background: none;
_padding-left: 0px;
_margin-left: -0px;
_padding-top: 36px;
_margin-top: -36px;
_filter: progid:DXImageTransform.Microsoft.AlphaImageLoader(sizingMethod=crop,src="sprite.png");
}
.cusicon_4 {
background: url(sprite.png) -36px -36px no-repeat;
width: 100%;
height: 100%;
_background: none;
_padding-left: 36px;
_margin-left: -36px;
_padding-top: 36px;
_margin-top: -36px;
_filter: progid:DXImageTransform.Microsoft.AlphaImageLoader(sizingMethod=crop,src="sprite.png");
}
.cusicon_5 {
background: url(sprite.png) -72px -0px no-repeat;
width: 100%;
height: 100%;
_background: none;
_padding-left: 72px;
_margin-left: -72px;
_padding-top: 0px;
_margin-top: -0px;
_filter: progid:DXImageTransform.Microsoft.AlphaImageLoader(sizingMethod=crop,src="sprite.png");
}
</style>
</head>
<body>
<ul>
<li class="active">
<div class="cusicon cusicon_1"></div>
<div class="tip">
发布项目
</div>
<div class="time">2015-04-02</div>
</li>
<li class="active">
<div class="cusicon cusicon_2"></div>
<div class="tip">
设计师报名
</div>
<div class="time"></div>
</li>
<li>
<div class="cusicon cusicon_3"></div>
<div class="tip">
选择设计师/备案
</div>
<div class="time"></div>
</li>
<li>
<div class="cusicon cusicon_4 last"></div>
<div class="tip">
确认完工
</div>
<div class="time"></div>
</li>
</ul>
</body>
</html>
五、用法：字体文字（例如ionicfont等）
六、按钮及布局特效
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title></title>
<style>
html{height: 100%;}
body{
margin: 0;
background-color: #f2f2f2;
display: flex;
justify-content: center;
align-items: center;
height: 100%;
}
.btn{
width: 570px;
height: 100px;
text-align: center;
line-height: 100px;
background-color: #fff;
}
.btn{position:relative;}
.btn:before, .btn:after {
z-index: -1;
position: absolute;
content: "";
bottom: 15px;
left: 5px;
width: 50%;
top: 80%;
max-width: 300px;
background: #777;
-webkit-box-shadow: 0 15px 10px #777;
-moz-box-shadow: 0 15px 10px #777;
box-shadow: 0 15px 10px #777;
-webkit-transform: rotate(-3deg);
-moz-transform: rotate(-3deg);
-o-transform: rotate(-3deg);
-ms-transform: rotate(-3deg);
transform: rotate(-3deg);
}
.btn:after {
-webkit-transform: rotate(3deg);
-moz-transform: rotate(3deg);
-o-transform: rotate(3deg);
-ms-transform: rotate(3deg);
transform: rotate(3deg);
right: 10px;
left: auto;
}
</style>
</head>
<body>
<div class="btn"> SIGN IN </div>
</body>
</html>
七、兼容性
- Chrome 2+
- Firefox 3.5+ (3.0 had partial support)
- Safari 1.3+
- Opera 9.2+
- IE8+ (with some minor bugs
- 几乎所有的移动浏览器
- 唯一真正的问题是没有获得支持的（不用奇怪）IE6和IE7
八、注意事项
- 由于伪元素不是真正的元素，所以不会出现在DOM中。因此，它们不是可用的。所以，不要使用伪元素生成内容，是您的网页的可用性和可访问性的关键。
- 由于伪元素难以维护和调试缓慢，所以不要用伪元素显示内容。
时间： 2024-10-21 17:37:45