This page checks (at various scales) what the minimum coordinate offset of a straight line's end point (relative to its start point) is for HTML canvas before rounding issues cause the line to misrender. Square line caps are used, and the line has an angle of -30 degrees to its user space x-axis. The expected rendering, therefore, is a square (one edge of which may be antialiased) rotated 30 degrees anticlockwise. As the coordinates of the of the line's endpoint approach the minimum offset the square may rotate away from the 30 degree angle. A small reduction more and the line will be treated as if it has zero length. The behaviour of platform libraries can be very different for zero length lines. Some throw the line away, while others render a square that is aligned with the user space x-axis. (In some cases it also depends on whether square or round line caps are being used, square being thrown away, round rendering as expected.)
In latest Mozilla Nightly this page can be used to check the underlying platform limits since canvas now uses Moz2D, and Moz2D (currently) just forwards everything to its backend.
Your results:
To validate these results you can use your browser's debugging tools to evaluate testRendering(scale, offset) and check whether the canvas element with the black border above shows a blue square rotated 30 degrees anticlockwise. (Note that rounding from double precision no the decimal numbers displayed can result in both the displayed minimum and first failure values being off by one in their least significant digit (potentially making them the same decimal value). If your efforts to validate fail, try again with the least significant digit of the minimum line offset incremented, and the least significant digit of the first failure's decremented.)