[css-animations] Clarify if synthesized 0% / 100% keyframes use the underlying animated style or the un-animated style

[birtles]

I believe Chrome and Firefox use the underlying animated style while other UAs use the un-animated style. The spec suggests we should use the un-animated style but it's not clear when/if that value should be snapshotted and this seems like less useful behavior than using the underlying animated style:

If a 0% or from keyframe is not specified, then the user agent constructs a 0% keyframe using the computed values of the properties being animated. If a 100% or to keyframe is not specified, then the user agent constructs a 100% keyframe using the computed values of the properties being animated.