yui/yui3 — comment on commit e1d2c90eed2334936210d1e1c6c53a43934ed517
File: src/button/css/cssbutton.css line 24
By ericf:

@derek is suppose to serve as the default color fo those browsers which do not understand RGBA. If I remember right, the order is significant here — IE will read the `color: rgba(0,0,0, 0.8);` rule but not apply it (my guess is it sees the string "rgb".)

Therefore to provide a fallback, the RGBA value must come first, followed by an IE-specific override.