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:first-child 选取第一个标签 :last-child 选取最后一个标签
:nth-child(2) 选取第二个标签 :nth-child(n) 选取第N个标签
:nth-child(2n) :nth-child(even) 选取偶数标签 :nth-child(2n+1) :nth-child(odd) 选取奇数标签
:nth-child(n+4) 选取大于等于4的标签 :nth-child(-n+4) 选取小于等于4的标签
:nth-child(3n+1) 自定义选择标签，n取0开始，隔二取一个标签
:nth-last-child(2) 选取倒数第二个标签 :nth-last-child(n) 选取倒数第N个标签
