Re: proposal: background-image-alpha

> Reasons: 1) GIF does not support grayscale (needed to serve alpha mask 
> purpose)

GIF does support grayscale to the same resolution as it supports
RGB (8 bit depth) but without the losses involved in paletisation,
as the palette can be a trivial, identity palette (0 = #000000,
1 = #010101.... 255 = #ffffff).