a {
  background-image: cross-fade(linear-gradient(rgb(1,2,3)));
}

b {
  background-image: cross-fade(0% linear-gradient(rgb(1,2,3)));
}

c {
  background-image: cross-fade(100% linear-gradient(rgb(1,2,3)), linear-gradient(rgb(3,2,1)));
}

d {
  background-image: cross-fade(75% cross-fade(linear-gradient(rgb(1,2,3))), linear-gradient(rgb(3,2,1)));
}