diff --git a/components/style/sharing/mod.rs b/components/style/sharing/mod.rs
index 314e9877083..d6148d16b49 100644
--- a/components/style/sharing/mod.rs
+++ b/components/style/sharing/mod.rs
@@ -815,7 +815,11 @@ impl<E: TElement> StyleSharingCache<E> {
         Some(candidate.element.borrow_data().unwrap().share_styles())
     }
 
-    /// Attempts to find an element in the cache with the given primary rule node and parent.
+    /// Attempts to find an element in the cache with the given primary rule
+    /// node and parent.
+    ///
+    /// FIXME(emilio): re-measure this optimization, and remove if it's not very
+    /// useful... It's probably not worth the complexity / obscure bugs.
     pub fn lookup_by_rules(
         &mut self,
         shared_context: &SharedStyleContext,
@@ -841,7 +845,15 @@ impl<E: TElement> StyleSharingCache<E> {
             if style.visited_rules() != visited_rules {
                 return None;
             }
-
+            // NOTE(emilio): We only need to check name / namespace because we
+            // do name-dependent style adjustments, like the display: contents
+            // to display: none adjustment.
+            if target.namespace() != candidate.element.namespace() {
+                return None;
+            }
+            if target.local_name() != candidate.element.local_name() {
+                return None;
+            }
             // Rule nodes and styles are computed independent of the element's
             // actual visitedness, but at the end of the cascade (in
             // `adjust_for_visited`) we do store the visitedness as a flag in
@@ -853,6 +865,7 @@ impl<E: TElement> StyleSharingCache<E> {
             // FIXME(jryans): This seems like it breaks the constant time
             // requirements of visited, assuming we get a cache hit on only one
             // of unvisited vs. visited.
+            // TODO(emilio): We no longer have such a flag, remove this check.
             if target.is_visited_link() != candidate.element.is_visited_link() {
                 return None;
             }
diff --git a/components/style/style_adjuster.rs b/components/style/style_adjuster.rs
index 8bbc40ca8c6..3983c780894 100644
--- a/components/style/style_adjuster.rs
+++ b/components/style/style_adjuster.rs
@@ -19,6 +19,14 @@ use properties::longhands::position::computed_value::T as Position;
 /// NOTE(emilio): If new adjustments are introduced that depend on reset
 /// properties of the parent, you may need tweaking the
 /// `ChildCascadeRequirement` code in `matching.rs`.
+///
+/// NOTE(emilio): Also, if new adjustments are introduced that break the
+/// following invariant:
+///
+///   Given same tag name, namespace, rules and parent style, two elements would
+///   end up with exactly the same style.
+///
+/// Then you need to adjust the lookup_by_rules conditions in the sharing cache.
 pub struct StyleAdjuster<'a, 'b: 'a> {
     style: &'a mut StyleBuilder<'b>,
 }