Safari and Transparent Borders
I had noticed this little bug on my own site. In the footer, there's a 5px border with the colour set using rgba. In Safari, it's as if the semi-transparent borders overlap each other in the corners and the values are compounded. This creates little squares in the corner of my squares.
Not quite what you'd expect. Firefox and Opera (10.5; I didn't test in 10.10) render this as you'd expect, with a consistent colour surrounding the block.
In testing some other border handling, I noticed that the overlapping only seemed to happen when the border colour was the same on all sides. If the border colour is the same but the border width is different, you'd still get the overlapping values in the corner.
Now, how far apart do the colour values need to be before it reverts to a different way of rendering the borders: generally 3/1000th of a difference.
With the following CSS, the borders render closer to expected in Safari:
You can still notice a slight line at each edge as I suspect some anti-aliasing is at play. This is consistent between both Safari and Chrome, since they're both based on Webkit.