Only one root element paints background.

[danfickle]

This means that html with a background-color or background-image overrides body with same.
Test cases at com.github.neoflyingsaucer.test.BackgroundTest.testBackgroundColorOnBodyAndHtml() and com.github.neoflyingsaucer.test.BackgroundTest.testBackgroundImageOnBodyAndHtml()

See also #33

[danfickle]

I think this may actually be two bugs. The first is that an empty body is not painting at all. The second is that in continuous mode (non print) the body background doesn't paint even if the body has content.

As an empty body is an unlikely scenario and the non-print continuous mode is not our primary use case, I've removed the critical label for now.