fix alpha(opacity=n) support

test/css/css.css

@@ -66,6 +66,7 @@ p + h1 {
   background-image: url(images/image.jpg);
   background: -webkit-gradient(linear, left top, left bottom, from(red), to(blue));
   margin: ;
+  filter: alpha(opacity=100);
 }
 #important {
   color: red !important;

test/css/variables.css

@@ -19,3 +19,6 @@
 .variable-names {
   name: 'hello';
 }
+.alpha {
+  filter: alpha(opacity=42);
+}