diff --git a/testing/web-platform/tests/shadow-dom/Element-interface-attachShadow-custom-element.html b/testing/web-platform/tests/shadow-dom/Element-interface-attachShadow-custom-element.html
index b59460e17b97..5d862c49244b 100644
--- a/testing/web-platform/tests/shadow-dom/Element-interface-attachShadow-custom-element.html
+++ b/testing/web-platform/tests/shadow-dom/Element-interface-attachShadow-custom-element.html
@@ -14,6 +14,78 @@ test(() => {
   assert_true(document.createElement('my-custom').attachShadow({mode: "open"}) instanceof ShadowRoot);
 }, 'Element.attachShadow must create an instance of ShadowRoot for autonomous custom elements');
 
+test(() => {
+  assert_true(document.createElement('undefined-custom').attachShadow({mode: 'open'}) instanceof ShadowRoot);
+}, 'Element.attachShadow must create an instance of ShadowRoot for undefined autonomous custom elements');
+
+test(() => {
+  class ShadowDisabledElement extends HTMLElement {
+    static disabledFeatures = ['shadow'];
+  }
+
+  // No definition. attachShadow() should succeed.
+  let element = document.createElement('shadow-disabled-element');
+  element.attachShadow({mode: 'closed'});
+
+  // No definition and it's already a host.
+  assert_throws('NotSupportedError', () => {
+    element.attachShadow({mode: 'closed'});
+  }, 'No definition, host');
+
+  // The element has a definition, and it's already a host.
+  customElements.define('shadow-disabled-element', ShadowDisabledElement);
+  assert_throws('NotSupportedError', () => {
+    element.attachShadow({mode: 'closed'});
+  }, 'Definition, host');
+
+  // The element has a definition, and it's not a host.
+  assert_throws('NotSupportedError', () => {
+    document.createElement('shadow-disabled-element').attachShadow({mode: 'closed'});
+  }, 'Definition, not a host');
+}, 'Element.attachShadow for an autonomous custom element with ' +
+   'disabledFeatures=["shadow"] should throw a NotSupportedError');
+
+test(() => {
+  class ShadowDisabledHeadingElement extends HTMLHeadingElement {
+    static disabledFeatures = ['shadow'];
+  }
+
+  // No definition. attachShadow() should succeed.
+  let element = document.createElement('h2',
+      {is: 'shadow-disabled-heading-element'});
+  element.attachShadow({mode: 'closed'});
+
+  // No definition and it's already a host.
+  assert_throws('NotSupportedError', () => {
+    element.attachShadow({mode: 'closed'});
+  }, 'No definition, host.');
+
+  // The element has a definition, and it's already a host.
+  customElements.define('shadow-disabled-heading-element',
+      ShadowDisabledHeadingElement, {extends: 'h2'});
+  assert_throws('NotSupportedError', () => {
+    element.attachShadow({mode: 'closed'});
+  }, 'Definition, host');
+
+  // The element has a definition, and it's not a host.
+  let h2 = document.createElement('h2', {is: 'shadow-disabled-heading-element'});
+  assert_throws('NotSupportedError', () => {
+    h2.attachShadow({mode: 'closed'});
+  }, 'Definition, not a host');
+}, 'Element.attachShadow for a customized built-in element with ' +
+   'disabledFeatures=["shadow"] should throw a NotSupportedError');
+
+test(() => {
+  class CapitalShadowDisabledElement extends HTMLElement {
+    static disabledFeatures = ['SHADOW'];
+  }
+
+  customElements.define('capital-shadow-disabled-element', CapitalShadowDisabledElement);
+
+  // Test fails if this throws
+  document.createElement('capital-shadow-disabled-element').attachShadow({mode: 'open'});
+}, 'Element.attachShadow for a custom element with disabledFeatures=["SHADOW"] should not throw a NotSupportedError');
+
 class MyCustomizedBuiltinElement extends HTMLInputElement {
 }