diff --git a/components/style_derive/to_css.rs b/components/style_derive/to_css.rs
index 19c7c1ac2bc..40e219c374b 100644
--- a/components/style_derive/to_css.rs
+++ b/components/style_derive/to_css.rs
@@ -140,11 +140,19 @@ fn derive_variant_fields_expr(
         Some(pair) => pair,
         None => return quote! { Ok(()) },
     };
+    if attrs.field_bound {
+        let ty = &first.ast().ty;
+        // TODO(emilio): IntoIterator might not be enough for every type of
+        // iterable thing (like ArcSlice<> or what not). We might want to expose
+        // an `item = "T"` attribute to handle that in the future.
+        let predicate = if attrs.iterable {
+            parse_quote!(<#ty as IntoIterator>::Item: style_traits::ToCss)
+        } else {
+            parse_quote!(#ty: style_traits::ToCss)
+        };
+        cg::add_predicate(where_clause, predicate);
+    }
     if !attrs.iterable && iter.peek().is_none() {
-        if attrs.field_bound {
-            let ty = &first.ast().ty;
-            cg::add_predicate(where_clause, parse_quote!(#ty: style_traits::ToCss));
-        }
         let mut expr = quote! { style_traits::ToCss::to_css(#first, dest) };
         if let Some(condition) = attrs.skip_if {
             expr = quote! {
diff --git a/components/style_traits/owned_slice.rs b/components/style_traits/owned_slice.rs
index f6068365360..36ba3162e59 100644
--- a/components/style_traits/owned_slice.rs
+++ b/components/style_traits/owned_slice.rs
@@ -93,12 +93,6 @@ impl<T: Sized> OwnedSlice<T> {
         ret
     }
 
-    /// Iterate over all the elements in the slice taking ownership of them.
-    #[inline]
-    pub fn into_iter(self) -> impl Iterator<Item = T> + ExactSizeIterator {
-        self.into_vec().into_iter()
-    }
-
     /// Convert the regular slice into an owned slice.
     #[inline]
     pub fn from_slice(s: &[T]) -> Self
@@ -109,6 +103,16 @@ impl<T: Sized> OwnedSlice<T> {
     }
 }
 
+impl<T> IntoIterator for OwnedSlice<T> {
+    type Item = T;
+    type IntoIter = <Vec<T> as IntoIterator>::IntoIter;
+
+    #[inline]
+    fn into_iter(self) -> Self::IntoIter {
+        self.into_vec().into_iter()
+    }
+}
+
 impl<T> Deref for OwnedSlice<T> {
     type Target = [T];