[css-values-4] Remove issue about infectious NaN (in favor of just making it infectious in all functions), and add explanation of behavior divergence from JS. Fixes w3c#4161.

Author: tabatkins

css-values-4/Overview.bs

@@ -2972,15 +2972,40 @@ Argument Ranges</h4>
 
 	(See [[#calc-type-checking]] for details on how [=math functions=] handle NaN and infinities.)
 
-	Note: All of these behaviors are intended to match the "standard" definitions of these functions
-	as implemented by most programming languages,
-	in particular as implemented in JS.
-
-	Issue: Per JS, hypot(∞, NaN) yields ∞, and pow(NaN, 0) yields 1.
-	This violates the standard calculation rules of NaNs being fully infective.
-	Do we want to carry that behavior over as well?
-	Or maybe specifically undefine it,
-	since it's just error behavior?
+	<div class=note>
+		All of these behaviors are intended to match the "standard" definitions of these functions
+		as implemented by most programming languages,
+		in particular as implemented in JS.
+
+		The only divergences from the behavior of the equivalent JS functions
+		are that NaN is "infectious" in <em>every</em> function,
+		forcing the function to return NaN if any argument calculation is NaN.
+
+		<details highlight=js>
+			<summary>Details of the JS Behavior</summary>
+
+			There are two cases in JS where a NaN is not "infectious"
+			to the math function it finds itself in:
+
+			* <code>Math.hypot(Infinity, NaN)</code> will return <code>Infinity</code>.
+			* <code>Math.pow(NaN, 0)</code> will return <code>1</code>.
+
+			The logic appears to be that,
+			if you replace the NaN with <em>any</em> Number,
+			the return value will be the same.
+			However, this logic is not applied consistently to the <code>Math</code> functions:
+			<code>Math.max(Infinity, NaN)</code> returns <code>NaN</code>, not <code>Infinity</code>;
+			the same is true of <code>Math.min(-Infinity, NaN)</code>.
+
+			Because this is an error corner case,
+			JS isn't consistent on the matter,
+			and NaN recognition/handling of [=calculations=]
+			is likely done at a higher CSS level
+			rather than in the internal math functions anyway,
+			consistency in CSS was chosen to be more important,
+			so all functions were defined to have "infection" NaN.
+		</details>
+	</div>
 
 
 <h3 id='calc-syntax'>