Corners gradient

This function is not in CSS spec, but works as the same principle as the conic-gradient demo.

.el {
  background: linear-gradient(to right, deeppink, cyan);
}
.el::before {
  content: '';
  position: absolute;
  top: 0; left: 0; right: 0; bottom: 0;
  border-radius: inherit;
  background: linear-gradient(to right, yellow, orangered);
  mask: linear-gradient(#0000, #000);
}
@supports (background: paint(id, foo)) {
  .el.is-loaded {
    background: paint(corners-gradient, deeppink, cyan, orangered, yellow);
  }
  .el.is-loaded::before {
    opacity: 0;
  }
}