CSS

background-image linear gradient and border radius

.rounded-corners-gradient-borders {
  width: 300px;
  height: 80px;
  border: double 4px transparent;
  border-radius: 80px;
  background-image: linear-gradient(white, white), radial-gradient(circle at top left, #f00,#3020ff);
  background-origin: border-box;
  background-clip: content-box, border-box;
}