diff --git a/components/script/dom/element.rs b/components/script/dom/element.rs
index dcc89638e8a..6dc7b7c3147 100644
--- a/components/script/dom/element.rs
+++ b/components/script/dom/element.rs
@@ -484,6 +484,9 @@ impl Element {
             return Err(Error::InvalidState);
         }
 
+        self.upcast::<Node>()
+            .set_flag(NodeFlags::IS_IN_SHADOW_TREE, true);
+
         // Steps 4, 5 and 6.
         Ok(self
             .shadow_root
diff --git a/components/script/dom/node.rs b/components/script/dom/node.rs
index 33c05aa2a5a..ba3298cb071 100644
--- a/components/script/dom/node.rs
+++ b/components/script/dom/node.rs
@@ -182,6 +182,9 @@ bitflags! {
 
         /// Whether this element has already handled the stored snapshot.
         const HANDLED_SNAPSHOT = 1 << 8;
+
+        // Whether this node participates in a shadow tree.
+        const IS_IN_SHADOW_TREE = 1 << 9;
     }
 }
 
@@ -265,8 +268,10 @@ impl Node {
         self.children_count.set(self.children_count.get() + 1);
 
         let parent_in_doc = self.is_in_doc();
+        let parent_in_shadow_tree = self.is_in_shadow_tree();
         for node in new_child.traverse_preorder() {
             node.set_flag(NodeFlags::IS_IN_DOC, parent_in_doc);
+            node.set_flag(NodeFlags::IS_IN_SHADOW_TREE, parent_in_shadow_tree);
             // Out-of-document elements never have the descendants flag set.
             debug_assert!(!node.get_flag(NodeFlags::HAS_DIRTY_DESCENDANTS));
             vtable_for(&&*node).bind_to_tree(parent_in_doc);
@@ -464,6 +469,10 @@ impl Node {
         self.flags.get().contains(NodeFlags::IS_IN_DOC)
     }
 
+    pub fn is_in_shadow_tree(&self) -> bool {
+        self.flags.get().contains(NodeFlags::IS_IN_SHADOW_TREE)
+    }
+
     /// Returns the type ID of this node.
     pub fn type_id(&self) -> NodeTypeId {
         match *self.eventtarget.type_id() {