The rules are different regarding the geometry of a shape when clipping and masking. For example, a clip-path does not take into account the stroke of the shape used for clipping. It is however, used when masking.
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The test is passed if there are two identical darkblue circles at the top of the illustration, and below those two circles, two more circles should appear. They are of lighter appearance, the one on the left has a darker and thick stroke.