How to Curve the hypotenuse of a triangle with CSS

Question

I would like to curve the hypotenuse side of this triangle div/background with CSS, so the result is something like this:

Answer 1

background can do it:

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

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