When implementing shape-outside in Firefox in Bug 1309467, I found the calculation of corner radii by margin-box and border-radius might need more clarification. Quote from the spec:

If the ratio of border-radius/margin is 1 or more, then the margin box corner radius is border-radius + margin. If the ratio of border-radius/margin is less than 1, then the margin box corner radius is border-radius + (margin * (1 + (ratio-1)^3)).

My questions mainly related to the ratio < 1 case.

1. Suppose ratio < 1 and margin > 0. Why do we need to adjust margin by margin * (1 + (ratio-1)^3) which makes the margin even smaller? It seems border-radius + margin is fine.
2. Suppose margin < 0, and we allow negative margin to reduce the final corner radii from border-radius. However based on the formula, it makes the final corner radii bigger! For example, suppose border-radius is 10px and margin is -5px, so the ratio is -2. The final corner radii is 10 + (-5) * (1 + (-3)^3) = 140. So it seems the spec needs some clarification when margin < 0. Again simple border-radius + margin seems fine with negative margin. If we do allow negative margin, the spec should explicit say that the corner radii is the larger of 0 and the final result.

cc @astearns @dbaron