<div></div>

div {
  width: 200px;
  height: 200px;
  border: solid 10px transparent;
  border-radius: 1em;
  background-image:
    linear-gradient(white, white),
    linear-gradient(to right, #2196F3, #673AB7, #9C27B0, #E91E63);
  background-origin: border-box;
  background-clip: content-box, border-box;
}