后台可以做到：

.box {
  width: 200px;
  height: 150px;
  background: radial-gradient(180% 180% at right -65% top -65%, transparent 99%, red )
}

<div class="box"></div>

另一种方法是，在覆盖了你需要的部分之后，用pseudo：匹配你所处的背景：
虽然我更喜欢@Temani Afif上面的回答。

#curved-triangle::after {
    content: "";
    position: absolute;
    top: -100px;
    left: 0px;
    background: #fff;
    width: 200px;
    height: 200px;
    border-radius: 100%;
}
#curved-triangle {
    width: 0;
    height: 0;
    border-left: 0px solid transparent;
    border-right: 100px solid transparent;
    border-bottom: 100px solid red;
    position: relative;
}

<div id="curved-triangle"></div>