SVG 1.2 Tiny has viewport-fill I'm not sure how widely implemented this property is though as most browsers are targetting SVG 1.1 at this time. Opera implements it FWIW.
A more cross-browser solution currently would be to stick a <rect>
element with width and height of 100% and fill="red" as the first child of the <svg>
element, for example:
<rect width="100%" height="100%" fill="red"/>
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