[css-values-4] Define how negative zero works in math functions. Finally closes #545.

Author: tabatkins

css-values-4/Overview.bs

@@ -1831,7 +1831,7 @@ Type Checking</h4>
 
 	Division by zero is possible,
 	which introduces certain complications.
-	[=Math functions=] follow simplified versions of IEEE-754 semantics for these operations:
+	[=Math functions=] follow IEEE-754 semantics for these operations:
 
 	* Dividing a positive value by zero produces +∞.
 	* Dividing a negative value by zero produces −∞.
@@ -1849,14 +1849,70 @@ Type Checking</h4>
 		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=])
+	Additionally,
+	IEEE-754 introduces the concept of "negative zero",
+	which must be tracked within a calculation
+	and between nested calculations:
+
+	* Negative zero
+		(0<sup>-</sup>)
+		can be produced literally by negating a zero
+		(''-0''),
+		or by a multiplication or division that produces zero
+		with exactly one negative argument
+		(such as ''-5 * 0'' or ''1 / (-1 / 0)'').
+
+		Note: Note that,
+		outside of [=math functions=],
+		''-0'' just produces a "standard" zero,
+		identical to ''0''--
+		CSS as a whole doesn't recognize the concept of signed zeros.
+		Negative zeros also don't escape a [=math function=];
+		as detailed below,
+		they're "censored" away into an "unsigned" zero.
+	* ''-0 + -0''
+		or ''-0 - 0''
+		produces 0<sup>-</sup>.
+		All other additions or subtractions that would produce a zero
+		produce 0<sup>+</sup>.
+	* Multiplying or dividing 0<sup>-</sup> with a positive number
+		(including 0<sup>+</sup>)
+		produces a negative result
+		(either 0<sup>-</sup> or −∞),
+		while multiplying or dividing 0<sup>-</sup> with a negative number
+		produces a positive result.
+
+		(In other words,
+		multiplying or dividing with 0<sup>-</sup>
+		follows standard sign rules.)
+	* When comparing 0<sup>+</sup> and 0<sup>-</sup>,
+		0<sup>-</sup> is less than 0<sup>+</sup>.
+		For example, ''min(0, -0)'' must produce 0<sup>-</sup>,
+		''max(0, -0)'' must produce 0<sup>+</sup>,
+		and ''clamp(0, -0, 1)'' must produce 0<sup>+</sup>.
+
+	If a <dfn export>top-level calculation</dfn>
+	(a [=math function=] not nested 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.
+	If a [=top-level calculation=] would produce 0<sup>-</sup>,
+	it instead produces the standard "unsigned" zero.
+
+	<div class=example>
+		For example, ''calc(-5 * 0)'' produces an unsigned zero--
+		the calculation resolves to 0<sup>-</sup>,
+		but as it's a [=top-level calculation=],
+		it's then censored to an unsigned zero.
+
+		On the other hand, ''calc(1 / calc(-5 * 0))'' produces −∞,
+		same as ''calc(1 / (-5 * 0))''--
+		the inner calc resolves to 0<sup>-</sup>,
+		and as it's not a [=top-level calculation=],
+		it passes it up unchanged to the outer calc to produce −∞.
+		If it was censored into an unsigned zero,
+		it would instead produce +∞.
+	</div>
+
 
 	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