[css-values-5] Define if() processing, add a note. w3c#10064

Author: fantasai

css-values-5/Overview.bs

@@ -1343,6 +1343,18 @@ Conditional Value Selection: the ''if()'' notation</h3>
 	The <dfn value for="if()">else</dfn> keyword represents
 	a condition that is always true.
 
+	To <dfn export>[=resolve an arbitrary substitution function|resolve an if() function=]</dfn>,
+	return the <<declaration-value>>?
+	associated with the first <<if-condition>> that is true;
+	if none are true,
+	return nothing (an empty token stream).
+
+	Note: Unlike using ''@media''/''@supports''/''@container'' rules,
+	which just ignore their contents when they're false
+	and let the cascade determine what values otherwise apply,
+	declarations with ''if()'' do not roll back the cascade if the conditions are false;
+	any fallback values must be provided inline.
+
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