radial-gradient
Button with radial gradient background
Ellipse, yellow (center) to green (corners) [equivalent]
Circle, yellow (center) to green (corners)
Ellipse, red (center), yellow, green (corners)
Red (bottom-left), yellow, green (top-right)
Repeating radial gradients
Ellipse (top-left, closest-side), red, yellow, green [equivalent]
Circle (top-left, closest-side), red, yellow, green [equivalent]
Ellipse (top-left, closest-corner), red, yellow, green
Circle (top-left, closest-corner), red, yellow, green
Sharp color transitions for testing anti-aliasing
Edge cases, should be green, possibly with a tiny ellipse.