[css-color-5] Pass the normalization flag.

Author: tabatkins

css-color-5/Overview.bs

@@ -271,12 +271,13 @@ Mixing Colors: the ''color-mix()'' Function {#color-mix}
 	To <dfn export>calculate a color-mix()</dfn>:
 
 	1. [=Normalize mix percentages=] from the list of [=mix items=] passed to the function,
+		with the "forced normalization" flag set to true,
 		letting |items| and |leftover| be the result.
-	2. Let |alpha mult| be <code>1 - |leftover|</code>,
-		interpreting |leftover| as a number between 0 and 1.
-	3. If |alpha mult| is 0,
+	2. If |leftover| is 100%,
 		return [=transparent black=],
 		converted to the specified interpolation <<color-space>>.
+	3. Let |alpha mult| be <code>1 - |leftover|</code>,
+		interpreting |leftover| as a number between 0 and 1.
 	4. If |items| is length 1,
 		set |color| to the color of that sole item.