This test has a gradient with gradientUnits='objectBoundingBox' which is a fade from black to white. The gradient is used for the stroke of a line. Vertical and horizontal lines don't have a boundingbox, since they are one-dimensional, even though the stroke-width makes it look like they should have a boundingbox with non-zero width and height. See the coordinate chapter, last paragraph of 7.11.
The left rectangle has four 'line' elements rotated in different ways. The stroke for the lines have a green solid stroke fallback which should be used if the gradient should be ignored. The right rectangle is the same as the left rectangle except that the stroke paintservers don't have a fallback specified.
The test is passed if