The table has four cells with white, black, dark green and pale yellow backgrounds. Each copy of the test image should be composited against the color of background that it finds itself against.
If each image is composited against the same color (for example all grey, or all black), that is a bug. You could also try the HTML 4.0 with CSS version to check that compositing against colors specified in a style sheet work correctly.
The following images show correct and incorrect compositing: