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

.el {
  background: linear-gradient(to bottom right, deeppink, transparent 50%, orangered),
             linear-gradient(to top right, yellow, transparent 50%, cyan) #888;
}
@supports (background: paint(id)) {
  .el.is-loaded {
    background: paint(corners-gradient, deeppink, cyan, orangered, yellow);
  }
}