cssbattle.css

<div></div>
<style>
  body {
    background: #293462;
    margin: 0;
  }
  div {
    width: 200px;
    height: 200px;
    background: #FFF1C1;
    position: relative;
    &:after, &:before {
      background: #FE5F55;
      box-sizing: border-box;
      content: '';
      position: absolute;
    }
    &:after {
      height: 100%;
      width: 100px;
      left: 100%;
      border-bottom: solid 50px #A64942;
    }
    &:before {
      height: 100px;
      width: 100%;
      top: 100%;
      left: 0;
      border-right: solid 50px #A64942;
      z-index: 1;
    }
  }
</style>