2 files changed, 13 insertions, 0 deletions
diff --git a/components/script/dom/customelementregistry.rs b/components/script/dom/customelementregistry.rs
index bfd0622dbfb..68186da108c 100644
--- a/components/script/dom/customelementregistry.rs
+++ b/components/script/dom/customelementregistry.rs
@@ -58,6 +58,7 @@ pub(crate) enum CustomElementState {
     Failed,
     #[default]
     Uncustomized,
+    Precustomized,
     Custom,
 }
 
@@ -924,6 +925,8 @@ fn run_upgrade_constructor(
         let _ac = JSAutoRealm::new(*cx, constructor.callback());
         let args = HandleValueArray::empty();
         // Step 8.2. Set element's custom element state to "precustomized".
+        element.set_custom_element_state(CustomElementState::Precustomized);
+
         if unsafe {
             !Construct1(
                 *cx,
diff --git a/components/script/dom/htmlelement.rs b/components/script/dom/htmlelement.rs
index 346b15ef009..6c8a29c4f22 100644
--- a/components/script/dom/htmlelement.rs
+++ b/components/script/dom/htmlelement.rs
@@ -13,6 +13,7 @@ use script_layout_interface::QueryMsg;
 use style::attr::AttrValue;
 use style_dom::ElementState;
 
+use super::customelementregistry::CustomElementState;
 use crate::dom::activation::Activatable;
 use crate::dom::attr::Attr;
 use crate::dom::bindings::codegen::Bindings::CharacterDataBinding::CharacterData_Binding::CharacterDataMethods;
@@ -618,6 +619,15 @@ impl HTMLElementMethods<crate::DomTypeHolder> for HTMLElement {
             return Err(Error::NotSupported);
         }
 
+        // Step 6: If this's custom element state is not "precustomized" or "custom",
+        // then throw a "NotSupportedError" DOMException.
+        if !matches!(
+            element.get_custom_element_state(),
+            CustomElementState::Precustomized | CustomElementState::Custom
+        ) {
+            return Err(Error::NotSupported);
+        }
+
         if self.is_form_associated_custom_element() {
             element.init_state_for_internals();
         }