Right now when no value for <box> is set margin-box is used. However that is only valid for shape-outside, for shape-inside and webkit-clipPath it should be content-box.
Created attachment 219768 [details] Patch
Comment on attachment 219768 [details] Patch Clear review flags for now until we are sure we get the right solution for basic shape interpolation, the spec is still a bit in flux. Keep the bug open as a reminder to revisit this.
Fixed as part of moving layout box into shape value. See bug 127200.