body {
  margin: 0;
  display: flex;
  align-items: center;
  justify-content: center;
  height: 100vh;
}

#color-wheel {
  height: 100vmin;
  width: 100vmin;
}

#color-wheel {
  border-radius: 50%;
  background: conic-gradient(
    in hsl longer hue,
    hsl(0deg 100% 50%),
    hsl(360deg 100% 50%)
  );
}
/* For browsers that don't support hue interpolation methods */
@supports not (background: conic-gradient(in hsl longer hue)) {
  #color-wheel {
    background: conic-gradient(red, yellow, green, cyan, blue, magenta, red);
  }
}