利用border属性和tranform变化,元素背景透明
:after {content: ""; position: absolute; top: 12px; right: -12px; margin: 20px; border-right: 1px solid #757575; border-bottom: 1px solid #757575; width: 10px; height: 10px; transform: rotate(-45deg);-o-transform: rotate(-45deg); -webkit-transform: rotate(-45deg); -moz-transform: rotate(-45deg); -ms-transform: rotate(-45deg);｝
