diff --git a/components/style/gecko/pseudo_element_definition.mako.rs b/components/style/gecko/pseudo_element_definition.mako.rs
index fd65de440b4..f11d46f59f3 100644
--- a/components/style/gecko/pseudo_element_definition.mako.rs
+++ b/components/style/gecko/pseudo_element_definition.mako.rs
@@ -15,6 +15,8 @@ pub enum PseudoElement {
     % endfor
 }
 
+/// Important: If you change this, you should also update Gecko's
+/// nsCSSPseudoElements::IsEagerlyCascadedInServo.
 <% EAGER_PSEUDOS = ["Before", "After", "FirstLine", "FirstLetter"] %>
 <% TREE_PSEUDOS = [pseudo for pseudo in PSEUDOS if pseudo.is_tree_pseudo_element()] %>
 <% SIMPLE_PSEUDOS = [pseudo for pseudo in PSEUDOS if not pseudo.is_tree_pseudo_element()] %>