this is a topic for people who are very sensitive to pixel errors. And there is one in the circle mark for the last stone played. With the latest Firefox on my full HD screen it is clearly visible and looks like a (hardware) pixel error. Here are two screenshots (scaled to 300% to make it more clear, but it is visible in the original size too):
With Chrome the antialiasing isn’t perfect, but it is ok and there is no one pixel error. On my iPad the circle looks perfect, probably because of the higher resolution.