The optional <<'border-radius'>> argument(s)
define rounded corners for the inset rectangle
using the 'border-radius' shorthand syntax.

It's not specified and I believe it should whether border-radius percentage values should be calculated based on inset rectangle or its outer box.

#example {
  width: 100px;
  height: 300px;
  background: red;
  clip-path: xywh(0% 0% 100% 33.3% round 50%)
}