lucy24 - 11:18 pm on Nov 21, 2013 (gmt 0)
One thing I found while looking it up yesterday is that if a browser can't do alpha channels it is supposed to
:: shuffling papers ::
If RGBA values are not supported by a user agent, they should be treated like unrecognized values per the CSS forward compatibility parsing rules ([CSS21], Chapter 4). RGBA values must not be treated as simply an RGB value with the opacity ignored.
Their italics. That's tucked away in the color module [w3.org], not in Backgrounds and Borders.
Unlike most rules about Conforming User Agents, I suspect this one really will work. A browser that doesn't do alpha channels probably won't even recognize the "rgba" formulation, so then it defaults to the "if you don't understand it, ignore it" CSS rule.
The part I can't figure out is whether you can make a "first choice, second choice" formulation, in the same way that you can offer a comma-delimited list of fonts.