Probably not a practical one, for the same reasons that discourage CSS polyfills. I think this feature is aimed not at developers who have to support legacy browsers but those who want to use newer CSS standards in self updating browsers without having to wait for the laggards. Safari is (was?) often criticized for being very slow to implement adopted CSS standards so that might have been the impetus. Once Houdini Paint/Layout/Animation APIs lands and enough clients update, devs can switch to Houdini and use any new/experimental CSS standards that has an open source implementation.