<div></div>

 * {
      box-sizing: border-box
    }


    div {
      width: 100px;
      height: 100px;
      border: 1px dotted gray;
      background: radial-gradient(at top left, gray 11px, red 12px, red 14px, transparent 15px) no-repeat right bottom / 15px 15px;
    }