You should see black circles in the boxes. IE (emulated through excanvas) and Chrome on Windows (and maybe others) fail the first test. They seem to have trouble drawing arcs with a start angle 0 and an end angle nearly but not exactly 2*PI. It is strange behavior since IE doesn't even implement the canvas tag.

Here is an arc that starts at 0 and ends at 2 * PI - 0.001

Here is an arc that starts at 0 and ends at exactly 2 * PI.

Here is a nearly closed arc starting at 1 and ending at 1 + 2 * PI - 0.0001