@si Barring research I think they're just wrong.
The argument for "user confusion" when the button sounds like nonsense and the rest of the arguments could be applied to transparent buttons. And will apply if transparency becomes a common UI element.
If going from grey to coloured actually does confuse people then the correct solution, IMO, would be to use muted colours and lowered contrast on disabled buttons. That way you can also control the colours better than by just adjusting the opacity.
@si Or making it so disabled buttons have a background colour which is closer to the page's background colour.
Or if the buttons contrast heavily with the background have the text and button colours swapped around and darkening/lightening them, depending on if the background is dark or light respectively.
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