[css-values-5] Add first-valid(). w3c#5055

Author: tabatkins

css-values-5/Overview.bs

@@ -521,6 +521,45 @@ Representing An Entire Property Value: the <<whole-value>> type</h3>
 	However, some functions may restrict
 	what a <<whole-value>> argument can include.
 
+<h3 id="first-valid">
+Selecting the First Supported Value: the ''first-valid()'' notation</h3>
+
+	CSS supports progressive enhancement with its forward-compatible parsing:
+	authors can declare the same property multiple times in a style rule,
+	using different values each time,
+	and a CSS UA will automatically use the last one that it understands
+	and throw out the rest.
+	This principle, together with the ''@supports'' rule,
+	allows authors to write stylesheets that work well
+	in old and new UAs simultaneously.
+
+	However, using ''var()''
+	(or similar substitution functions that resolve after parsing)
+	thwarts this functionality;
+	CSS UAs must assume any such property is valid at parse-time.
+
+	The <dfn>first-valid()</dfn> [=functional notation=]
+	inlines the fallback behavior
+	intrinsic to parsing declarations.
+	Unlike most notations,
+	it can accept any valid or invalid syntax in its arguments,
+	and represents the first value among its arguments
+	that is supported (parsed as valid) by the UA
+	as the whole value of the property it's used in.
+
+	<pre class=prod>
+		<dfn>&lt;first-valid()></dfn> = first-valid( <<declaration-value>> [ ';' <<declaration-value>> ]* )
+	</pre>
+
+	If none of the arguments represent a valid value for the property,
+	the property is [=invalid at computed-value time=].
+
+	''first-valid()'' is a <<whole-value>>.
+
+	Issue: Should this have a different name?
+	We didn't quite decide on it during the resolution to add this.
+
+
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