注意:1.CSS中a:hover 必须在a:link和 a:visited后出现,才能正常显示效果!(a:hover MUST come after a:link and a:visited in the CSS definition in order to be effective!);
      2.CSS中a:active必须出现在a:hover定义后才能有效果!(Note: a:hover MUST come after a:link and a:visited in the CSS definition in order to be effective!)


a:link       初始状态
a:visited   已经访问过的
a:hover     把鼠标放上去时
a:active    鼠标点击时