diff --git a/shadow-dom/declarative/element-internals-shadowroot.tentative.html b/shadow-dom/declarative/element-internals-shadowroot.tentative.html
index 0f01cc41acf153..8469a6c1f1ffd6 100644
--- a/shadow-dom/declarative/element-internals-shadowroot.tentative.html
+++ b/shadow-dom/declarative/element-internals-shadowroot.tentative.html
@@ -41,4 +41,73 @@
   assert_true(constructed);
 }, 'ElementInternals.shadowRoot allows access to closed shadow root');
 
+test(() => {
+  let constructed = false;
+  const element = document.createElement('x-1');
+  assert_throws_dom('NotSupportedError', () => element.attachInternals(),'attachInternals cannot be called before definition exists');
+  customElements.define('x-1', class extends HTMLElement {
+    constructor() {
+      super();
+      assert_true(!!this.attachInternals());
+      constructed = true;
+    }
+  });
+  assert_false(constructed);
+  assert_throws_dom('NotSupportedError', () => element.attachInternals(),'attachInternals cannot be called before constructor');
+  customElements.upgrade(element);
+  assert_true(constructed);
+  assert_throws_dom('NotSupportedError', () => element.attachInternals(),'attachInternals already called');
+}, 'ElementInternals cannot be called before constructor, upgrade case');
+
+test(() => {
+  let constructed = false;
+  const element = document.createElement('x-2');
+  customElements.define('x-2', class extends HTMLElement {
+    constructor() {
+      super();
+      // Don't attachInternals() here
+      constructed = true;
+    }
+  });
+  assert_throws_dom('NotSupportedError', () => element.attachInternals(),'attachInternals cannot be called before constructor');
+  assert_false(constructed);
+  customElements.upgrade(element);
+  assert_true(constructed);
+  assert_true(!!element.attachInternals(),'After the constructor, ok to call from outside');
+}, 'ElementInternals *can* be called after constructor, upgrade case');
+
+test(() => {
+  let constructed = false;
+  customElements.define('x-3', class extends HTMLElement {
+    constructor() {
+      super();
+      assert_true(!!this.attachInternals());
+      constructed = true;
+    }
+  });
+  const element = document.createElement('x-3');
+  assert_true(constructed);
+  assert_throws_dom('NotSupportedError', () => element.attachInternals(), 'attachInternals already called');
+}, 'ElementInternals cannot be called after constructor calls it, create case');
+
+test(() => {
+  let constructed = false;
+  const element = document.createElement('x-5');
+  customElements.define('x-5', class extends HTMLElement {
+    static disabledFeatures = [ 'internals' ];
+    constructor() {
+      super();
+      assert_throws_dom('NotSupportedError', () => this.attachInternals(), 'attachInternals forbidden by disabledFeatures, constructor');
+      constructed = true;
+    }
+  });
+  assert_false(constructed);
+  assert_throws_dom('NotSupportedError', () => element.attachInternals(), 'attachInternals forbidden by disabledFeatures, pre-upgrade');
+  customElements.upgrade(element);
+  assert_true(constructed);
+  assert_throws_dom('NotSupportedError', () => element.attachInternals(), 'attachInternals forbidden by disabledFeatures, post-upgrade');
+}, 'ElementInternals disabled by disabledFeatures');
+
+
+
 </script>