[css-color-5] sum to zero needs to be handled

Author: svgeesus

css-color-5/Overview.bs

@@ -130,7 +130,8 @@ Mixing colors: the ''color-mix()'' function {#color-mix}
 	2. If both percentages are omitted, they each default to 50% (an equal mix of the two colors).
 	3. Otherwise, if <var>p2</var> is omitted, it becomes 100% - <var>p1</var>
 	4. Otherwise, if <var>p1</var> is omitted, it becomes 100% - <var>p2</var>
-	5. Otherwise, if both are provided but do not add up to 100%,
+	5. If the percentages sum to zero <em>do something, tbd</em>
+	6. Otherwise, if both are provided but do not add up to 100%,
 	they are scaled accordingly so that they add up to 100%.
 	This means that
 	<var>p1</var> becomes <var>p1</var> / (<var>p1</var> + <var>p2</var>)