[css-color-4] set chroma to 0 when converting powerless hue to missing

Author: romainmenke

css-color-4/Overview.bs

@@ -1293,6 +1293,11 @@ will have a [=powerless=] hue component.
 	have an <em>extremely small</em> chroma rather than precisely ''0%'';
 	as a result, the hue component is [=powerless=].
 
+	When changing a powerless hue component to a [=missing component=],
+	the chroma (or other measure of colorfulness, such as saturation in ''hsl'') is set to zero
+	to avoid amplifying floating-point noise.
+	In the case of ''hwb'' set ''b'' to ''100 - w'' or if either ''b'' or ''w'' is missing set the other component to ''100''.
+
 <h3 id=parse-color>
 Parsing a <<color>> Value</h3>