Miscellaneous build / tidy fixes.

components/style_traits/Cargo.toml

@@ -16,7 +16,7 @@ gecko = []
 [dependencies]
 app_units = "0.7"
 bitflags = "1.0"
-cssparser = "0.27"
+cssparser = "0.28"
 euclid = "0.20"
 lazy_static = "1"
 malloc_size_of = { path = "../malloc_size_of" }

components/style_traits/values.rs

@@ -118,6 +118,16 @@ where
     }
 }
 
+impl ToCss for () {
+    #[inline]
+    fn to_css<W>(&self, _: &mut CssWriter<W>) -> fmt::Result
+    where
+        W: Write,
+    {
+        Ok(())
+    }
+}
+
 /// A writer tailored for serialising CSS.
 ///
 /// Coupled with SequenceWriter, this allows callers to transparently handle