如何在html5和css3中创建这个形状？

Question

如何在html5和css3中创建此形状？

Answer 1

#big {
        position: relative;
        background: black;
        width: 600px;
        height: 600px;        
    }
    #in {
        position: absolute;
        background: black;
        width: 280px;
        height: 280px;
        bottom:0;
        right: 0;
        border-top: 20px solid white;
    border-left: 20px solid white;
    }

<div id="big">
    <div id="in"></div>
</div>