--- a/servo/components/style/keyframes.rs
+++ b/servo/components/style/keyframes.rs
@@ -68,16 +68,29 @@ impl KeyframePercentage {
         Ok(percentage)
     }
 }
 
 /// A keyframes selector is a list of percentages or from/to symbols, which are
 /// converted at parse time to percentages.
 #[derive(Debug, PartialEq)]
 pub struct KeyframeSelector(Vec<KeyframePercentage>);
+
+impl ToCss for KeyframeSelector {
+    fn to_css<W>(&self, dest: &mut W) -> fmt::Result where W: fmt::Write {
+        let mut iter = self.0.iter();
+        iter.next().unwrap().to_css(dest)?;
+        for percentage in iter {
+            write!(dest, ", ")?;
+            percentage.to_css(dest)?;
+        }
+        Ok(())
+    }
+}
+
 impl KeyframeSelector {
     /// Return the list of percentages this selector contains.
     #[inline]
     pub fn percentages(&self) -> &[KeyframePercentage] {
         &self.0
     }
 
     /// A dummy public function so we can write a unit test for this.
@@ -103,30 +116,24 @@ pub struct Keyframe {
     /// Note that `!important` rules in keyframes don't apply, but we keep this
     /// `Arc` just for convenience.
     pub block: Arc<Locked<PropertyDeclarationBlock>>,
 }
 
 impl ToCssWithGuard for Keyframe {
     fn to_css<W>(&self, guard: &SharedRwLockReadGuard, dest: &mut W) -> fmt::Result
     where W: fmt::Write {
-        let mut iter = self.selector.percentages().iter();
-        try!(iter.next().unwrap().to_css(dest));
-        for percentage in iter {
-            try!(write!(dest, ", "));
-            try!(percentage.to_css(dest));
-        }
+        self.selector.to_css(dest)?;
         try!(dest.write_str(" { "));
         try!(self.block.read_with(guard).to_css(dest));
         try!(dest.write_str(" }"));
         Ok(())
     }
 }
 
-
 impl Keyframe {
     /// Parse a CSS keyframe.
     pub fn parse(css: &str, parent_stylesheet: &Stylesheet)
                  -> Result<Arc<Locked<Self>>, ()> {
         let error_reporter = NullReporter;
         let context = ParserContext::new(parent_stylesheet.origin,
                                          &parent_stylesheet.url_data,
                                          &error_reporter,