[css-properties-values-api] Remove Issue 1. (#883)

Generating said token stream is now described by § 4.5. Substitution.

Author: andruud

css-properties-values-api/Overview.bs

@@ -698,10 +698,6 @@ will interpolate as specified in [[!css3-animations]].
 Otherwise, the interpolation falls back to the default 50% flip described in
 [[!css3-animations]].
 
-Issue: Intermediate interpolated results of animations on custom properties must
-be able to generate a token stream representing their value. We should ensure that
-this is standard across implementations to avoid interop issues.
-
 Conditional Rules {#conditional-rules}
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