JSFiddle - React, Tailwind, and code Playground

HTML

<div class="parent">
<div class="cornor">
</div>
</div>

CSS

.cornor {
  position: fixed;
  top:0;
  left:0;
  border: 1px solid red;
  width: 100px; /* 设置容器宽度 */
  height: 100px; /* 设置容器高度 */
  background: radial-gradient(circle at 100% 100%, transparent 100px, #cccccc 100px);
}