<div style=" margin-top: 100px;width: 300px;height: 300px;position: relative;border: 1px solid #000000">
<div style="width:0; height:0; border-width:30px; border-style:solid;
border-color:transparent transparent #000 transparent;
position: absolute;top: -60px;left: 100px;
"></div>
<em style="display: block;width:0; height:0; border-width:29px; border-style:solid;
border-color:transparent transparent #fff transparent;position: absolute;top: -57px;left: 101px;"></em>
</div>
核心是一个三角形去拼接矩形框,另一个三角形去镂空,三角形通过 “ width:0; height:0; border-width:30px; border-style:solid;border-color:transparent transparent #000 transparent” 实现。