只需
:after以与
#alertlist li:hover选择器相同的方式将其追加到选择
#alertlist li.selected器即可：
#alertlist li.selected:after, #alertlist li:hover:after{ position:absolute; top: 0; right:-10px; bottom:0; border-top: 10px solid transparent; border-bottom: 10px solid transparent; border-left: 10px solid #303030; content: "";}
