// CSS代码
#myId:before {
content: "hello world!";
display: block;
width: 100px;
height: 100px;
background: red;
}
// HTML代码
<div id="myId"></div>
// JS代码
var myIdElement = document.getElementById("myId");
var beforeStyle = window.getComputedStyle(myIdElement, ":before");
console.log(beforeStyle); // [CSSStyleDeclaration Object]
console.log(beforeStyle.width); // 100px
console.log(beforeStyle.getPropertyValue("width")); // 100px
console.log(beforeStyle.content); // "hello world!"

// CSS代码
.red::before { 
content: "red"; 
color: red; 
}
.green::before { 
content: "green"; 
color: green;
}
// HTML代码
<div class="red">内容内容内容内容</div>
// jQuery代码
$(".red").removeClass('red').addClass('green');

document.styleSheets[0].addRule('.red::before','color: green'); // 支持IE document.styleSheets[0].insertRule('.red::before { color: green }', 0); // 支持非IE的现代浏览器

var style = document.createElement("style"); 
document.head.appendChild(style); 
sheet = style.sheet; 
sheet.addRule('.red::before','color: green'); // 兼容IE浏览器
sheet.insertRule('.red::before { color: green }', 0); // 支持非IE的现代浏览器

$('<style>.red::before{color:green}</style>').appendTo('head');

var latestContent = "修改过的内容";
var formerContent = window.getComputedStyle($('.red'), '::before').getPropertyValue('content'); document.styleSheets[0].addRule('.red::before','content: "' + latestContent + '"'); document.styleSheets[0].insertRule('.red::before { content: "' + latestContent + '" }', 0);

// CSS代码
.red::before {
content: attr(data-attr);
color: red;
}
// HTML代码
<div class="red" data-attr="red">内容内容内容内容</div>
// JacaScript代码
$('.red').attr('data-attr', 'green');

a:after { content: "after content"; }

a:after { content: attr(href); }
a:after { content: attr(data-attr); }

a::before { content: url(logo.png); }

h:before { counter-increment: chapter; cotent: "Chapter " counter(chapter) ". " }

.clear-fix { *overflow: hidden; *zoom: 1; }
.clear-fix:after { display: table; content: ""; width: 0; clear: both; }

// CSS代码
a {
position: relative;
display: inline-block;
text-decoration: none;
color: #000;
font-size: 32px;
padding: 5px 10px;
}
a::before, a::after { 
content: "";
transition: all 0.2s;
}
a::before { 
left: 0;
}
a::after { 
right: 0;
}
a:hover::before, a:hover::after { 
position: absolute;
}
a:hover::before { content: "\5B"; left: -20px; }
a:hover::after { content: "\5D"; right: -20px; }
// HTML代码
<a href="#">我是个超链接</a>

// CSS代码
.tooltip {
position: relative;
display: inline-block;
padding: 5px 10px;
background: #80D4C8;
}
.tooltip:before {
content: "";
display: block;
position: absolute;
left: 50%;
margin-left: -5px;
bottom: -5px;
width: 0; 
height: 0; 
border-left: 5px solid transparent;
border-right: 5px solid transparent;
border-top: 5px solid #80D4C8;
}
// HTML代码
<div class="tooltip">I'm a tooltip.</div>