[CSS21] background propagation

The rules for propagating the background from BODY to the canvas
say:

   # For HTML documents whose root HTML element has computed values
   # of 'transparent' for 'background-color' and 'none' for
   # 'background-image', user agents must instead use the computed
   # value of those properties from that HTML element's first BODY
   # element child when painting backgrounds for the canvas, and
   # must not paint a background for that BODY element.

There's an error here, it says "those properties" which implies
'background-color' and 'background-image' only. However the rule
should propagate /all/ background properties, not just those two.

I've fixed that error in css3-background. Perhaps we could just copy
out the text from there.
   http://dev.w3.org/csswg/css3-background/#special-backgrounds

~fantasai