A skew transformation along the y-axis is equivalent to the matrix [1 tan(a) 0 1 0 0], which has the effect of skewing Y coordinates by angle 'a'. The test overlays a group of black graphics elements with a 'skewY' transform specified on top of an identical group of red elements with the equivalent 'matrix' transform and vice versa.
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Test passes if there is no red visible on the page.