In the URL, the vertical borders are rendered as 3px wide rectangles, but they should be 1px wide. It seems like antialiasing doesn't kick in at that angle, and the coordinates get rounded to two integers 3px apart.
This is especially problematic when slowly easing in or out of 0deg rotation, because then the borders momentarily jump right at the end or the beginning of the transition.
"It seems like antialiasing doesn't kick in at that angle'
We don't antialias border line drawing. It caused problematic artifacts at border joins and stuff and made CSS artwork using borders differ from other browsers.
(In reply to comment #2)
> We don't antialias border line drawing.
I don't mean WebKit, I mean Core Graphics. See the antialiasing in this example:
data:text/html,div style="width: 50px; height: 50px; border: 1px solid; -webkit-transform: rotate(30deg);"></div>