Not described anywhere so far, but here is the idea: As we work on new levels of selectors, new possibilities are being introduced, and of course, they will not be simultaneously supported in all browsers. This means combining newish selectors with old ones using commas isn't safe, as old browsers would discard everything, not just the bit they don't understand. This encourages authors to duplicate declaration blocks instead of using comma separated selectors. Invalidating per comma separated group, rather than the entire selector, like media queries do, would fix this.Florian Rivoal, 9 Feb 2012, 23:07:50
That said, glazou has already mostly talked me out of this, on the argument that this would break the web. Previously failing selectors may start to match, and activate unexpected declaration blocks.
If we can do this without breaking the web, I think it would be nice, but as this is not critical, maybe it isn't worth the risk.