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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
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+++ b/testing/web-platform/tests/shadow-dom/Element-interface-attachShadow-custom-element.html
@@ -0,0 +1,101 @@
+<!DOCTYPE html>
+<title>Shadow DOM: Attaching a ShadowRoot for custom elements</title>
+<meta name="author" title="Hayato Ito" href="mailto:hayato@chromium.org">
+<link rel="help" href="https://dom.spec.whatwg.org/#dom-element-attachshadow">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script>
+class MyAutonomousCustomElement extends HTMLElement {
+}
+
+customElements.define('my-custom', MyAutonomousCustomElement);
+
+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 get disabledFeatures() { return ['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_dom('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_dom('NotSupportedError', () => {
+    element.attachShadow({mode: 'closed'});
+  }, 'Definition, host');
+
+  // The element has a definition, and it's not a host.
+  assert_throws_dom('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 get disabledFeatures() { return ['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_dom('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_dom('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_dom('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 get disabledFeatures() { return ['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 {
+}
+
+customElements.define('my-input', MyCustomizedBuiltinElement, { extends: 'input' });
+
+test(() => {
+  assert_throws_dom('NotSupportedError', () => {
+    document.createElement('input', {is: 'my-input'}).attachShadow({mode: "open"});
+  });
+}, 'Element.attachShadow must throw a NotSupportedError for customized built-in elements');
+</script>
+</body>
+</html>