Examine the test images in the left column below. To determine how your browser handles PNG transparency, find the images on the right that look the most similar.
Not all possible results are shown; there are too many combinations of background colors and shapes of the opaque region. However, I intend to include every result that actually occurs in a mainstream browser. If I am missing any, please let me know.
It’s come to my attention that my images which show how alpha transparency should look are not quite perfect in regard to precisely how transparent they are at various points. Rather than try to modify this page to test gamma correction issues as well, I’ve created a separate test page for that.
(via PNG transparency test)