diff --git a/components/style/parallel.rs b/components/style/parallel.rs
index 889f7f0a15b..b82727657c3 100644
--- a/components/style/parallel.rs
+++ b/components/style/parallel.rs
@@ -73,6 +73,12 @@ fn top_down_dom<N, C>(unsafe_nodes: UnsafeNodeList,
         // Possibly enqueue the children.
         let mut children_to_process = 0isize;
         for kid in node.children() {
+            // Trigger the hook pre-adding the kid to the list. This can (and in
+            // fact uses to) change the result of the should_process operation.
+            //
+            // As of right now, this hook takes care of propagating the restyle
+            // flag down the tree. In the future, more accurate behavior is
+            // probably going to be needed.
             context.pre_process_child_hook(node, kid);
             if context.should_process(kid) {
                 children_to_process += 1;