nth-child,nth-last-child,after,before,tab-highlight-color,first-child,last-child
nth-child:定义第几个元素或者是奇数或者是偶数,或者满足某个数字倍数的dom的样式如li:nth-child(3n),结果如下,li:nth-child(2)结果如下nth-last-child道理和nth-child一样,只不过是反过来p:nth-last-child(4n){color:red;}before和after就是在元素前面和后面添加上文本,一般结合content来使用tab