//获取伪元素
// 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!

//更改样式:
 1 // 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');