<div></div>

@import url("https://fonts.googleapis.com/css2?family=Exo:wght@600&display=swap");

* {
  margin: 0;
  padding: 0;
  box-sizing: border-box;
}

body {
  width: 100vw;
  min-height: 100vh;
  font-family: "Exo", Arial, sans-serif;
  background-color: #557;
  color: #fff;
  padding: 1rem;
  display: flex;
  justify-content: center;
  align-items: center;
}

div {
  width: min(100%, 50vw);
  height: max(40vh, 200px);
  background-color: #09f;
}

:root {
  --w: 760px;
  --max-radius: 8px;
  --min-radius: 0px;
  --radius: (100vw - var(--w));

  --responsive-radius: clamp(
    var(--min-radius),
    var(--radius) * 1000,
    var(--max-radius)
  );
}

div {
  border-radius: var(--responsive-radius);
}