For image types that do not support an alpha channel, the image must be
composited onto a solid black background using the source-over operator,
and the resulting image must be the one used to create the data: URL.
 See:
http://www.whatwg.org/specs/web-apps/current-work/multipage/the-canvas-element.html#dom-canvas-todataurl
Mozilla
    Bug 650720
Output:
    
        | Type: | image/png | image/jpeg | image/bmp (24 bpp)
 | image/bmp (32 bpp)
 | 
    
        | rgba(128, 255, 128, 0.5) |  |  |  |  | 
    
        | rgba(255, 128, 128, 0.75) |  |  |  |  | 
    
        | rgba(128, 128, 255, 0.25) |  |  |  |  | 
    
        | rgba(255, 255, 255, 1.0) |  |  |  |  | 
    
        | rgba(255, 255, 255, 0) |  |  |  |  | 
    
        | rgba(0, 0, 0, 1.0) |  |  |  |  | 
    
        | rgba(0, 0, 0, 0) |  |  |  |  |