二者的主要区别是是否创造了新元素
伪元素：（创建了’假‘元素） :before/::before :after/::after :first-letter/::first-letter :first-line/::first-line :section :placeholder
状态类伪类： :link :visited :hover :active :focus 结构类伪类 :not :first-child :last-child :only-child nth-child :nth-last-child 表单类伪类 input:checked input:disabled input:enabled .empty:empty 其他伪类
