handle , at toplevel

src/stylesheet.rs

@@ -295,6 +295,9 @@ impl<'a> StyleSheetParser<'a> {
                 c if c.is_control() => {
                     return Err(("expected selector.", self.lexer.next().unwrap().pos()).into());
                 }
+                ',' => {
+                    return Err(("expected \"{\".", self.lexer.next().unwrap().pos()).into());
+                }
                 _ => todo!("unexpected toplevel token: {:?}", kind),
             };
         }

tests/error.rs

@@ -82,3 +82,7 @@ error!(
     nothing_after_variable_in_style,
     "a {$a", "Error: expected \":\"."
 );
+error!(
+    toplevel_comma,
+    "a {},", "Error: expected \"{\"."
+);