css - 添加一个ID到:before Pseudo Class

Question

是否可以在 :before 伪类的内容中添加类或 ID？我知道这行不通，但是类似:

#menu {
    height: 100px;
    width: 100px;
}
#menu:before {
    class: "just-before-menu";
}
.just-before-menu {
    background: green;
    height: 100px;
    width: 100px;
}

Answer 1

The :before selector inserts content before the content of the selected element(s).

Use the content property to specify the content to insert.

#menu:before {
    content: '';
    background: green;
    height: 100px;
    width: 100px;
}

http://www.w3schools.com/cssref/sel_before.asp