clarify applicable value range

Author: romainmenke

css-color-4/Overview.bs

@@ -1296,15 +1296,17 @@ will have a [=powerless=] hue component.
 	When changing a powerless hue component to a [=missing component=],
 	the chroma (or other measure of colorfulness, such as saturation in ''hsl''; see [=analogous components=]) is set to zero
 	to avoid amplifying floating-point noise.
+	Negative values are not clamped to zero,
+	this value is only set to zero when it is larger than zero but smaller or equal to ε.
 	For ''hwb'' follow these steps instead:
 		<pre>
-		1. if W + B is greater than 99.999 and W + B is less than 100
-		2. if W and B are not missing
-			2.1. set B to 100 - W
-		3. else if W is not missing
-			3.1. set W to 100
-		4. else if B is not missing
-			4.1. set B to 100
+		1. if W + B is greater than or equal to ε and W + B is less than 100
+			1.1. if W and B are not missing
+				1.1.1. set B to 100 - W
+			1.2. else if W is not missing
+				1.2.1. set W to 100
+			1.3. else if B is not missing
+				1.3.1. set B to 100
 		</pre>
 
 <h3 id=parse-color>