As requested by Simon Fraser, I'm reporting this.
I'm using Webkit nightlies (6533.19.4, r78794) to test.
In Webkit there's a diagonal white line (a gap) between the black triangles. Gecko (nightlies) gets it right, so I added the -moz- variants so you can check how it looks there.
It doesn't seem to happen when the color stops are at 50%, but it does at 25%. Not sure what other numbers trigger it.
*** This bug has been marked as a duplicate of bug 54615 ***