[css-variables] Allow var(--a,) to be valid. Fixes w3c#6345

Author: tabatkins

css-variables-1/Overview.bs

@@ -425,6 +425,16 @@ Using Cascading Variables: the ''var()'' notation</h2>
 	<dfn>var()</dfn> = var( <<custom-property-name>> , <<declaration-value>>? )
 	</pre>
 
+	In an exception to the usual <a grammar lt=,>comma elision rules</a>,
+	which require commas to be omitted when they're not separating values,
+	a bare comma, with nothing following it,
+	must be treated as valid in ''var()'',
+	indicating an empty fallback value.
+
+	Note: That is, ''var(--a,)'' is a valid function,
+	specifying that if the ''--a'' custom property is invalid or missing,
+	the ''var()'' should be replaced with nothing.
+
 	The ''var()'' function can be used in place of any part of a value in any property on an element.
 	The ''var()'' function can not be used as
 	property names,
@@ -604,7 +614,7 @@ Invalid Variables</h3>
 
 		:   The property is a non-registered [=custom property=]
 		:   The property is a [=registered custom property=]
-		    with [=univerisal syntax definition|universal syntax=]
+		    with [=universal syntax definition|universal syntax=]
 		::  The computed value is the <a>guaranteed-invalid value</a>.
 
 		:   Otherwise