--- a/servo/components/style/properties/properties.mako.rs
+++ b/servo/components/style/properties/properties.mako.rs
@@ -1417,20 +1417,22 @@ impl UnparsedValue {
         custom_properties: Option<<&Arc<::custom_properties::CustomPropertiesMap>>,
         quirks_mode: QuirksMode,
     ) -> PropertyDeclaration {
         ::custom_properties::substitute(&self.css, self.first_token_type, custom_properties)
         .ok()
         .and_then(|css| {
             // As of this writing, only the base URL is used for property
             // values.
+            //
+            // FIXME(emilio): These bits are slightly fishy.
             let context = ParserContext::new(
                 Origin::Author,
                 &self.url_data,
-                None,
+                Some(CssRuleType::Style),
                 ParsingMode::DEFAULT,
                 quirks_mode,
             );
 
             let mut input = ParserInput::new(&css);
             Parser::new(&mut input).parse_entirely(|input| {
                 match self.from_shorthand {
                     None => longhand_id.parse_value(&context, input),