[css-mixins-1] Allow a mixin to be invoked without an arg list.

Author: tabatkins

css-mixins-1/Overview.bs

@@ -1133,7 +1133,7 @@ in place of the ''@apply'' rule itself.
 Its grammar is:
 
 <pre class="prod">
-<<@apply>> = @apply <<dashed-function>> [ { <<declaration-list>> } ]?;
+<<@apply>> = @apply [ <<dashed-ident>> | <<dashed-function>> ] [ { <<declaration-list>> } ]?;
 </pre>
 
 The ''@apply'' rule is only valid
@@ -1148,16 +1148,19 @@ adjust which properties and rules are active in a stylesheet
 before styles are applied.)
 
 The ''@apply'' rule applies the [=mixin=]
-named by the <<dashed-function>>'s name.
+named by the <<dashed-ident>> or the <<dashed-function>>'s name.
 If no such [=mixin=] exists,
 the ''@apply'' does nothing.
 
-The arguments passed to the <<dashed-function>>
+If passed a <<dashed-function>>,
+the arguments passed to the <<dashed-function>>
 are mapped to the [=mixin's=] arguments;
 if more arguments are passed than the length of the [=mixin's=] argument list,
 the ''@apply'' application does nothing.
 (Passing too few arguments is fine;
 the missing arguments take their default values instead.)
+A <<dashed-ident>> passes no arguments.
+(That is, ''@apply --foo;'' is identical to ''@apply --foo();''.)
 
 If the [=mixin=] declares a ''@contents'' parameter,
 and the ''@apply'' rule has a <<declaration-list>> block,