Actually, turns out that this a problem with PDF and SVG as well. For SVG, you can hack a <rect id="background" x="X0" y="Y0" width="100%" height="100%" style="fill:#000000"/> tag into the beginning to fill it. X0, Y0 are the upper left of the viewBox.
Actually, turns out that this a problem with PDF and SVG as well. For SVG, you can hack a <rect id="background" x="X0" y="Y0" width="100%" height="100%" style=" fill:#000000" /> tag into the beginning to fill it. X0, Y0 are the upper left of the viewBox.