伪元素

/*动态添加一个行元素*/
        /*before 前 div内容 after 后*/
        div:before,div:after {
            content: "after 伪元素";
            color: gray;
            width: 50px;
            height: 50px;
            display: block;
            background-color: yellow;
        }
        
        /*选中区域设置*/
        div::selection {
            background-color: red;
        }