Both produce:
WARNING: Fallback background-color (hex or RGB) should precede RGBA background-color. For older browsers that don't support RGBA, HSL, or HSLA, provide a fallback color. (fallback-colors) Browsers: IE6,IE7,IE8
If a browser cannot produce a semi-transparent background, you should also be able to remove the background altogether.
or
Both produce: WARNING: Fallback background-color (hex or RGB) should precede RGBA background-color. For older browsers that don't support RGBA, HSL, or HSLA, provide a fallback color. (fallback-colors) Browsers: IE6,IE7,IE8
If a browser cannot produce a semi-transparent background, you should also be able to remove the background altogether.