[css-values] More strictly define the infinity/NaN semantics for math functions.

Author: tabatkins

css-values-4/Overview.bs

@@ -1829,17 +1829,34 @@ Type Checking</h4>
 	If it can't [=CSSNumericValue/match=] any of these,
 	the [=math function=] is invalid.
 
-	As a further breaking change from the previous version of this specification,
-	division by zero is now allowed.
-	Implementations must follow IEEE-754 semantics for recognition and propagation
-	of infinities and NaN values,
-	including tracking the difference between +0 and -0.
-	(However, CSS treats both zeroes as the same,
-	if a [=math function=] resolves to one of them.)
-	Infinities are, by definition, out-of-range for any property,
-	and trigger appropriate clamping at used-value time per [[#calc-range]].
-	If a [=math function=] would resolve to NaN,
-	it instead resolves to positive infinity.
+	Division by zero is possible,
+	which introduces certain complications.
+	[=Math functions=] follow simplified versions of IEEE-754 semantics for these operations:
+
+	* Dividing a positive value by zero produces +∞.
+	* Dividing a negative value by zero produces −∞.
+	* Adding or subtracting ±∞ to anything produces the appropriate infinity,
+		unless a following rule would define it as producing NaN.
+	* Multiplying any value by ±∞ produces the appropriate infinity,
+		unless a following rule would define it as producing NaN.
+	* Dividing any value by ±∞ produces zero,
+		unless a following rule would define it as producing NaN.
+	* Dividing zero by zero,
+		dividing ±∞ by ±∞,
+		multiplying 0 by ±∞,
+		adding +∞ to −∞
+		(or the equivalent subtractions)
+		produces NaN.
+	* Any operation with at least one NaN argument produces NaN.
+
+	If a <dfn export>top-level calculations</dfn>
+	(an argument to ''min()'', ''max()'', or ''clamp()'',
+	or a ''calc()'' that's not nested directly inside of another [=math function=])
+	would produce a NaN,
+	it instead produces +∞.
+	Other calculations
+	(such as a ''calc()'' nested directly within a [=math function=])
+	can produce NaN as normal.
 
 	Note: Algebraic simplifications do not affect the validity of a [=math function=] or its resolved type.
 	For example, ''calc(5px - 5px + 10s)'' and ''calc(0 * 5px + 10s)'' are both invalid
@@ -1956,6 +1973,15 @@ Range Checking</h4>
 	Note: This requires all contexts accepting ''calc()''
 	to define their allowable values as a closed (not open) interval.
 
+	Note: By definition,
+	±∞ are outside the allowed range for any property,
+	and will clamp to the minimum/maximum value allowed.
+	Even for properties that explicitly allow ''infinity'' as a keyword value,
+	such as 'animation-iteration-count',
+	will end up clamping ±∞,
+	as [=math functions=] can't resolve to keyword values;
+	the <em>numeric</em> part of the property's syntax still has a minimum/maximum value.
+
 	Additionally, if a [=math function=] that resolves to <<number>>
 	is used somewhere that only accepts <<integer>>,
 	the [=computed value=] and [=used value=] are rounded to the nearest integer,