[css-color-5] Define edge cases of color-mix() with calc, by WG resolution. Fix #10772

Author: svgeesus

css-color-5/Overview.bs

@@ -2854,16 +2854,44 @@ followed by the specified <<color-space>> in all-lowercase,
 followed by ", ",
 followed by the first specified color,
 followed by a space,
-followed by the specified (un-normalized) first percentage (unless both percentages are 50%),
+followed by the serialization of the first percentage (see below)
 followed by ", ",
 followed by the second specified color,
-followed by the specified (un-normalized) second percentage
-(unless the two specified percentages add to 100%),
+followed by the serialization of the second percentage (see below),
 followed by ")".
 
-Following the principle of shortest serialization,
-the second percentage is typically omitted,
-even if explicitly specified.
+The serialization of the <strong>first</strong> percentage of a declared value of a ''color-mix()'' function is defined as:
+
+	- If BOTH the first percentage |p1| and second percentage |p2| are specified:
+		- if both |p1| equals 50% and |p2| equals 50%, nothing is serialized. <!-- (a) -->
+		- else, |p1| is serialized as is. <!-- (b) -->
+	- else if ONLY the first percentage |p1| is specified:
+		- if |p1| is equal to 50%, nothing is serialized. <!-- (c) -->
+		- else, |p1| is serialized as is. <!-- (d) -->
+	- else if ONLY the second percentage |p2| is specified:
+		- if |p2| equals 50%, nothing is serialized. <!--  (e) -->
+		- if |p2| is not ''calc()'', the value of 100% - |p2| is serialized. <!-- (f)-->
+		- else, nothing is serialized. <!-- (g) -->
+	- else if NEITHER is specified:
+		- nothing is serialized. <!-- (h) -->
+
+The serialization of the <strong>second</strong> percentage of a declared value of a ''color-mix()'' function is defined as:
+
+	- If BOTH the first percentage p1 and second percentages p2 are specified:
+		- if neither p1 nor p2 is calc(), and p1 + p2 equals 100%, nothing is serialized. <!-- (i) -->
+		- else, p2 is serialized as is. <!-- (j) -->
+	- else if ONLY the first percentage p1 is specified:
+		- nothing is serialized. <!-- (k) -->
+	- else if ONLY the second percentage p2 is specified:
+		- if p2 equals 50%, nothing is serialized. <!-- (l) -->
+		- if p2 is not calc(), nothing is serialized. <!-- (m) -->
+		- else, p2 is serialized as is. <!-- (n) -->
+	- else if NEITHER is specified:
+		- nothing is serialized. <!-- (o) -->
+
+Note: ''calc()'' values are consider to be unknown, 
+so are <strong>never</strong> equal 50%,
+and never sum with something else to equal 100%.
 
 <div class="example" id="ex-serial-specified-mix">
 	For example, the serialized declared value of