The following is an image with four different background colors, using plain-olde RGB hex values. At the corners, the border has a bevel-like effect.

The next image is using CSS 3’s new RGBa color values with alpha transparency, in white. In Firefox, it works as expected. In Safari/WebKit, the corners seem to be overlapping.

The final image is using the same RGBa technique, but with different colors. Note that the corners are beveled again on the different colors, but overlapping where same-color edges meet.

Reported on WebKit’s Bugzilla (21835)