<div class="box1"></div>

css:

.box1 {
  width: 300px;
  height: 300px;
  background-color: red;
  /* 左上方圆角半径 */
  border-top-left-radius: 300px;
  border-bottom-right-radius: 100px;
}

代码演示:

202101211611221218585629