Also, anti-aliased images contain a lot more colours than aliased ones, which puts pressure on the colour allocation strategy . Variable transparency is most easily implemented with a truecolour display, because if any image pixel may be mixed with the user agent's background colour or texture in any proportion, the total number of colours in a displayed image can become quite large. It can be implemented in a colour-scarce environment provided an off screen buffer is provided to composite the image before reducing it to the current palette. Recommendations Future Web user agents should permit as inline images formats that support variable transparency, to allow quality anti-aliased images that are independent of the background colour or texture. This will encourage information providers to migrate away from formats that only support binary transparency, such as GIF89a.
Overlays are only of use if one can also see what is being overlaid.