Now that we have user specified interpolation of gradient color stops, we should how to extend this to CSS Animations and WebAnimations. See https://bugs.webkit.org/show_bug.cgi?id=208465 for more thoughts on this.
<rdar://problem/87070224>
If I understand https://drafts.csswg.org/css-images-4/#linear-gradients correctly,<color-interpolation-method> is part of the linear-gradient property itself. If that's correct, then I don't think animation specifications need to address this, but the relevant spec should define how interpolation works with that keyword present or absent.