background-image:
-moz-linear-gradient(0 0 -45deg,
#ffff00,
#ff0000,
#00ff00,
#0000ff
);
background-image:
-moz-linear-gradient(0 0 -45deg,
#ffff00 20%,
#ff0000 40%,
#00ff00 60%,
#0000ff 80%
);
background-image:
-moz-linear-gradient(-200px -200px -45deg,
#ffff00 20%,
#ff0000 40%,
#00ff00 60%,
#0000ff 80%
);