Previously, the first `HTMLHtmlELement` in the given document would
always be used when determining whether to propagate background
properties to the body element. This meant the wrong root element was
used for SVG `foreignObject` elements, which could lead to a crash.
Previously, an SVG with width of zero would have am intrinsic aspect
ratio of zero. With this change, if an SVG has a width or height of
zero, the intrinsic aspect ratio is determined by the SVG's viewbox.
