Test to verify that the defs element is used as a container correctly.
In this test a fill is created which is solid green. The view should be a solid green rectangle centered in the viewport 100 pixels from from left,top and right,bottom. Also, in the defs sections there are rectangle defined, one to paint over the entire canvas with a red fill and the other to obscure most of the green rectangle.
The rendered picture should match the reference image, except for possible variations in the labelling text (per CSS2 rules).
A green rectangle should be visible, and no red.