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.
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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. For this sub-test to pass there must be three lines with solid green stroke, and one line (from bottom left to top right) with a gradient stroke, visible in the rectangle.
The right rectangle is the same as the left rectangle except that the stroke paintservers don't have a fallback specified. For this sub-test to pass only the line from bottom left to top right must be visible in the rectangle, and it must have a gradient stroke.