[css-values-4] Define how to serialize a math function that's infinit…

…e or NaN. Fixes #3768.

css-values-4/Overview.bs

@@ -3594,12 +3594,25 @@ Serialization</h3>
 			then serialize the value as normal
 			and return the result.
 
-		2. Let |s| be a [=string=] that initially contains
+		2. If |fn| represents an infinite or NaN value:
+			1. Let |s| be the [=string=] "calc(".
+			2. Create a numeric value
+				in the [=canonical unit=] for |fn|'s [=CSSNumericValue/type=]
+				(such as ''px'' for <<length>>),
+				with a value of 1 if |fn| represents +∞,
+				a value of -1 if |fn| represents −∞,
+				or a value of 0 if |fn| represents NaN.
+				Serialize this numeric value
+				and append it to |s|.
+			3. Append " / 0)" to |s|,
+				and return |s|.
+
+		3. Let |s| be a [=string=] that initially contains
 			the |fn|'s name
 			(such as "calc", "sin", etc)
 			followed by a "(" (open parenthesis).
 
-		3. For each comma-separated [=calculation=] argument of |fn|,
+		4. For each comma-separated [=calculation=] argument of |fn|,
 			[=serialize the calculation tree=].
 			If a result of this starts with a "(" (open parenthesis)
 			and ends with a ")" (close parenthesis),
@@ -3608,9 +3621,9 @@ Serialization</h3>
 			using ", " (comma followed by space),
 			then append the result to |s|.
 
-		4. Append ")" (close parenthesis) to |s|.
+		5. Append ")" (close parenthesis) to |s|.
 
-		5. Return |s|.
+		6. Return |s|.
 	</div>
 
 	<div algorithm>