For any pixel with a non-0 alpha, it is treated as having an alpha of 255 (or 1, in a schema where alpha is binary/boolean). On top of this, the 'background fill' value for any pixel with less than 255 alpha is white, causing a horrible border effect:

https://www.dropbox.com/s/3794m59uhvecxn6/png%20support%20is%20broken.png?dl=0