This is an adaptation of the "misc/backgrounds.html" file in the
Serenity image. It tests a lot of background-related properties that we
otherwise have no tests for.
This is based on the border-radius.html demo page with text and most
assets removed. This has to be a screenshot based test as there's not
really something else that can be used for comparison.
This also makes the test a little incomplete as things like text
overflow clipping are not tested, but I'd like to avoid this test being
too brittle.
This fixes the sizing of the arrow icons displayed to the left of
parent sections on a table of contents on Wikipedia articles, which
are sized using the equation `calc(max(0.75em, 12px))`. Now, the icon
will not expand past the edges of the box they are within, avoiding
clipping the edges of the arrows.
