Address review comments.

src/components/script/dom/document.rs

@@ -610,7 +610,8 @@ impl Document {
 
         // TODO: support the case if multiple elements
         // which haves same id are in the same document.
-        // This would use mangle(), but some dumbass removed it.
+        // FIXME https://github.com/mozilla/rust/issues/13195
+        //       Use mangle() when it exists again.
         match self.idmap.find_mut(&id) {
             Some(v) => {
                 *v = element.clone();