The CSS Values and Units Level 4 spec introduces a new mix() function: https://drafts.csswg.org/css-values-4/#interpolate. This is useful for blending especially in situations when we can't determine how to resolve values, for instance https://github.com/w3c/csswg-drafts/issues/2854 and our bug https://bugs.webkit.org/show_bug.cgi?id=246914.
<rdar://problem/102188082>