http://dev.w3.org/csswg/css3-background/#the-border-radius says that "a gradient is recommended for color transitions" between borders with a nonzero border-radius in that corner, but we don't implement that.

Ugh, I don't think we should; or at the very least, we should add another css property to control whether the transition occurs or not.

It's just a "recommended," but maybe you should bring that up with the committee?

This page have a good example for the transition, not ? http://www.owlfolio.org/htmletc/border-radius/

I'm flattered, but the example on that page was thrown together in 10 minutes with Illustrator and is not remotely precise enough to *define* the necessary gradient. Pixman does support conical gradients and has for quite some time (the code appears to have been added sometime in 2007) but Cairo doesn't expose it :-(

The recommendation to use a gradient for color transitions was obviously removed from the spec. See https://drafts.csswg.org/css-backgrounds-3/#changes-2009 (removed in https://github.com/w3c/csswg-drafts/commit/626d36526cd7a9e4a8f52723ca7a21c2b54f1d81). So, it's up to the user agent how to transition between the colors. Sebastian