--- a/servo/components/style/font_face.rs
+++ b/servo/components/style/font_face.rs
@@ -19,16 +19,17 @@ use parser::{ParserContext, ParserErrorC
 #[cfg(feature = "gecko")]
 use properties::longhands::font_language_override;
 use selectors::parser::SelectorParseErrorKind;
 use shared_lock::{SharedRwLockReadGuard, ToCssWithGuard};
 use std::fmt::{self, Write};
 use str::CssStringWriter;
 use style_traits::{Comma, CssWriter, OneOrMoreSeparated, ParseError};
 use style_traits::{StyleParseErrorKind, ToCss};
+use style_traits::values::SequenceWriter;
 use values::computed::font::FamilyName;
 #[cfg(feature = "gecko")]
 use values::specified::font::{SpecifiedFontFeatureSettings, SpecifiedFontVariationSettings};
 use values::specified::url::SpecifiedUrl;
 
 /// A source for a font-face rule.
 #[cfg_attr(feature = "servo", derive(Deserialize, Serialize))]
 #[derive(Clone, Debug, Eq, PartialEq, ToCss)]
@@ -44,25 +45,41 @@ impl OneOrMoreSeparated for Source {
     type S = Comma;
 }
 
 /// A `UrlSource` represents a font-face source that has been specified with a
 /// `url()` function.
 ///
 /// <https://drafts.csswg.org/css-fonts/#src-desc>
 #[cfg_attr(feature = "servo", derive(Deserialize, Serialize))]
-#[derive(Clone, Debug, Eq, PartialEq, ToCss)]
+#[derive(Clone, Debug, Eq, PartialEq)]
 pub struct UrlSource {
     /// The specified url.
     pub url: SpecifiedUrl,
     /// The format hints specified with the `format()` function.
-    #[css(skip)]
     pub format_hints: Vec<String>,
 }
 
+impl ToCss for UrlSource {
+    fn to_css<W>(&self, dest: &mut CssWriter<W>) -> fmt::Result where W: fmt::Write {
+        self.url.to_css(dest)?;
+        if !self.format_hints.is_empty() {
+            dest.write_str(" format(")?;
+            {
+                let mut writer = SequenceWriter::new(dest, ", ");
+                for hint in self.format_hints.iter() {
+                    writer.item(hint)?;
+                }
+            }
+            dest.write_char(')')?;
+        }
+        Ok(())
+    }
+}
+
 /// A font-display value for a @font-face rule.
 /// The font-display descriptor determines how a font face is displayed based
 /// on whether and when it is downloaded and ready to use.
 #[allow(missing_docs)]
 #[cfg_attr(feature = "servo", derive(Deserialize, Serialize))]
 #[derive(Clone, Copy, Debug, Eq, MallocSizeOf, Parse, PartialEq)]
 #[derive(ToComputedValue, ToCss)]
 pub enum FontDisplay {