If a PNG file has a aplpha plain composeed of only 0 or 255,
its transparency is ignored.

The attached file transparent1.png is a red sqaure framed with a green
line, where red pixels have 0 for alpha value and green pixels have 255
for alpha value.
transparent2.png is a similar image, but the alpha values for green
pixels are 254.

Red pixels in the both images have 0 for alpha value, so they should be
transparent and invisible, but transparent1.png is displayed as a red
square with a green frame in a emacs buffer.

transparent2.png is displayed as a white square with a green frame as
expected.

When they are displayed in a chrome browser, inner squares of the both
images are displayed as transparent.