This patch implements basic support for presentational hints, which are
old-school HTML attributes that affect style.
You add support for a presentational hint attribute by overriding
Element::apply_presentational_hints(StyleProperties&) and setting all
of the corresponding CSS properties as appropriate.
To make the background color fill the entire document, not just the
bounds of the <body> element's LayoutNode, we special-case it in the
HtmlView::paint_event() code for now. I'm not entirely sure what the
nicest solution would be, but I'm sure we'll discover it eventually.
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