[css3-background] background of canvas being partially opaque

css3-background defines what happens if the background given to the
canvas is the background-color 'transparent' [1]:
  # If the root's background-color value is 'transparent', the color
  # is UA dependent.

However, it does not define what happens if a partially opaque color
is used through the rgba() or hsla() functions.  ('transparent' is
really just a special case, where the a component is 0.)  It should.

I suggest that it should also specify that the behavior must be as
though it were composited against something (whether it's a
particular solid color or the contents of the window behind).

So I propose to replace the sentence quoted above with the
following:

  The root's background-color may not be completely opaque, for
  example, because it was specified using the 'transparent' keyword,
  the rgba() function, or the hsla() function.  This specification
  does not define what the canvas's background is drawn on top of.

-David

[1] http://www.w3.org/TR/2005/WD-css3-background-20050216/#the-background0

-- 
L. David Baron                                <URL: http://dbaron.org/ >
           Technical Lead, Layout & CSS, Mozilla Corporation