[css-values-4] Clarify that Quirks Mode has no effect on math-function parsing, but functions resolving to numbers might be okay due to quirks. w3c#4874

Author: tabatkins

css-values-4/Overview.bs

@@ -3852,6 +3852,17 @@ Type Checking</h3>
 	<<number>>s never become "length-like" in ''calc()''.
 	They always stay as <<number>>s.
 
+	Note: In Quirks Mode [[quirks]],
+	some properties that would normally only accept <<length>>s
+	are defined to also accept <<number>>s,
+	interpreting them as ''px'' lengths.
+	Like unitless zeroes,
+	this has no effect on the parsing or behavior of [=math functions=],
+	tho a [=math function=] that resolves to a <<number>> value
+	might become valid in Quirks Mode
+	(and have its result interpreted as a ''px'' length).
+
+
 <h3 id='calc-internal'>
 Internal Representation</h3>