div {
background: radial-gradient(circle 20% at 50% 50%, blue , red);
width: 1000px;
height: 1000px;
}
background: radial-gradient(circle 20px at 50% 50%, blue , red);
background: radial-gradient(circle at 50% 50%, blue 10% , red 20%);
background: radial-gradient(20% 20% at 50% 50%, blue, red);