[css-mixins-1] Make parentheses optional for mixins without parameters. #13015

Author: kbabbitt

css-mixins-1/Overview.bs

@@ -953,7 +953,7 @@ and a [=mixin body=].
 save that it lacks a [=custom function/return type=].)
 
 <pre class="prod def">
-	<<@mixin>> = @mixin <<function-token>> <<function-parameter>>#? )
+	<<@mixin>> = @mixin <<function-token>> ( <<function-parameter>># )?
 	{
 		<<declaration-rule-list>>
 	}