实现矩形凸三角

<style>
        .demo {
            width: 100px;
            height: 100px;
            background-color: #fff;
            border: 2px solid  #333;
            position: relative;
        }
        .demo:after,.demo:before {
            border: solid transparent;
            content: ' ';
            height: 0;
            left: 100%;
            position: absolute;
            width: 0;
        }
        .demo:after {
            border-width: 10px;
            border-left-color: #fff;
            top: 20px;
        }
        .demo:before {
            border-width: 12px;
            border-left-color: #000;
            top: 18px;
        }
    </style>


<div class="demo"></div>