an alpha-transparent border should be applied uniformly even when the object’s background is a gradient. In Webkit the top and bottom borders are alpha-transparency of the parent object, but the left and right borders are alpha-transparency of the object’s gradient background.
It seems there are a bunch of border issues, with this bug being an overview of many of them:
Fx 3.6/ Gecko 1.9.2 is fairly buggy in this, still (in short, rendering of the linked testcase is incorrect as well - the gradient should repeat into the border edge and affect the color of the border). That is fixed on Gecko trunk.
Created attachment 50689 [details]
Note also that the linked test case doesn't apply the same gradient for Gecko and WebKit (the WebKit gradient has repeat-x, the Gecko one has 'repeat' - the default).
Attached testcase corrects this (and the rendering is identical in Gecko 1.9.3a latest nightly and WebKit latest nightly, with the exception of the corners.
Thanks for adding the extra information Philippe, and for uploading a corrected test case. I’ve also added repeat-x to my test case
*** This bug has been marked as a duplicate of bug 21835 ***