[css-color-5] interpolation from/to currentColor, helps w3c#445

Author: svgeesus

css-color-5/Overview.bs

@@ -764,6 +764,9 @@ Issue: Should gamut mapping occur before or after interpolation?
 
 Issue(4928): How to handle achromatic interpolation?
 
+Interpolating to or from ''<color>/currentcolor'' is possible.
+The numerical value used for this purpose is the used value.
+
 Color space for interpolation {#interpolation-space}
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