diff --git a/testing/web-platform/tests/custom-elements/scoped-registry/ShadowRoot-innerHTML-upgrade.tentative.html b/testing/web-platform/tests/custom-elements/scoped-registry/ShadowRoot-innerHTML-upgrade.tentative.html
new file mode 100644
index 0000000000000..e21c9dd03326d
--- /dev/null
+++ b/testing/web-platform/tests/custom-elements/scoped-registry/ShadowRoot-innerHTML-upgrade.tentative.html
@@ -0,0 +1,116 @@
+<!DOCTYPE html>
+<meta name="author" title="Xiaocheng Hu" href="mailto:xiaochengh@chromium.org">
+<meta name="assert" content="Custom element constructors can re-enter with different definitions">
+<link rel="help" href="https://wicg.github.io/webcomponents/proposals/Scoped-Custom-Element-Registries">
+<link rel="help" href="https://github.com/WICG/webcomponents/issues/969">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+
+<div id="testdiv"></div>
+
+<script>
+class TestAutonomous extends HTMLElement {};
+class TestCustomizedBuiltIn extends HTMLParagraphElement {};
+
+function attachShadowForTest(t, registry) {
+  const host = document.createElement('div');
+  const shadow = host.attachShadow({mode: 'open', registry});
+  document.body.appendChild(host);
+  t.add_cleanup(() => host.remove());
+  return shadow;
+}
+
+test(t => {
+  const registry = new CustomElementRegistry;
+  registry.define('test-element', TestAutonomous);
+
+  const shadow = attachShadowForTest(t, registry);
+  shadow.innerHTML = '<test-element></test-element>';
+  assert_true(shadow.firstChild instanceof TestAutonomous, 'target tree scope');
+
+  // Verify that it doesn't pollute other tree scopes.
+  const shadow2 = attachShadowForTest(t);
+  shadow2.innerHTML = '<test-element></test-element>';
+  assert_false(shadow2.firstChild instanceof TestAutonomous, 'tree scope without registry');
+
+  const shadow3 = attachShadowForTest(t, new CustomElementRegistry);
+  shadow3.innerHTML = '<test-element></test-element>';
+  assert_false(shadow3.firstChild instanceof TestAutonomous, 'tree scope with different registry');
+
+  t.add_cleanup(() => testdiv.firstChild.remove());
+  testdiv.innerHTML = '<test-element></test-element>';
+  assert_false(testdiv.firstChild instanceof TestAutonomous, 'main document');
+}, 'Upgrade into autonomous custom element when inserted via innerHTML');
+
+test(t => {
+  const registry = new CustomElementRegistry;
+  const shadow = attachShadowForTest(t, registry);
+  shadow.innerHTML = '<test-element></test-element>';
+
+  const shadow2 = attachShadowForTest(t);
+  shadow2.innerHTML = '<test-element></test-element>';
+
+  const shadow3 = attachShadowForTest(t, new CustomElementRegistry);
+  shadow3.innerHTML = '<test-element></test-element>';
+
+  t.add_cleanup(() => testdiv.firstChild.remove());
+  testdiv.innerHTML = '<test-element></test-element>';
+
+  registry.define('test-element', TestAutonomous);
+
+  // Elements in the target tree scope should be upgraded.
+  assert_true(shadow.firstChild instanceof TestAutonomous, 'target tree scope');
+
+  // Verify that it doesn't pollute other tree scopes.
+  assert_false(shadow2.firstChild instanceof TestAutonomous, 'tree scope without registry');
+  assert_false(shadow3.firstChild instanceof TestAutonomous, 'tree scope with different registry');
+  assert_false(testdiv.firstChild instanceof TestAutonomous, 'main document');
+}, 'Upgrade into autonomous custom element when definition is added');
+
+test(t => {
+  const registry = new CustomElementRegistry;
+  registry.define('test-element', TestCustomizedBuiltIn, {extends: 'p'});
+
+  const shadow = attachShadowForTest(t, registry);
+  shadow.innerHTML = '<p is="test-element"></p>';
+  assert_true(shadow.firstChild instanceof TestCustomizedBuiltIn, 'target tree scope');
+
+  // Verify that it doesn't pollute other tree scopes.
+  const shadow2 = attachShadowForTest(t);
+  shadow2.innerHTML = '<p is="test-element"></p>';
+  assert_false(shadow2.firstChild instanceof TestCustomizedBuiltIn, 'tree scope without registry');
+
+  const shadow3 = attachShadowForTest(t, new CustomElementRegistry);
+  shadow3.innerHTML = '<p is="test-element"></p>';
+  assert_false(shadow3.firstChild instanceof TestCustomizedBuiltIn, 'tree scope with different registry');
+
+  t.add_cleanup(() => testdiv.firstChild.remove());
+  testdiv.innerHTML = '<p is="test-element"></p>';
+  assert_false(testdiv.firstChild instanceof TestCustomizedBuiltIn, 'main document');
+}, 'Upgrade into customized built-in element when inserted via innerHTML');
+
+test(t => {
+  const registry = new CustomElementRegistry;
+  const shadow = attachShadowForTest(t, registry);
+  shadow.innerHTML = '<p is="test-element"></p>';
+
+  const shadow2 = attachShadowForTest(t);
+  shadow2.innerHTML = '<p is="test-element"></p>';
+
+  const shadow3 = attachShadowForTest(t, new CustomElementRegistry);
+  shadow3.innerHTML = '<p is="test-element"></p>';
+
+  t.add_cleanup(() => testdiv.firstChild.remove());
+  testdiv.innerHTML = '<p is="test-element"></p>';
+
+  registry.define('test-element', TestCustomizedBuiltIn, {extends: 'p'});
+
+  // Elements in the target tree scope should be upgraded.
+  assert_true(shadow.firstChild instanceof TestCustomizedBuiltIn, 'target tree scope');
+
+  // Verify that it doesn't pollute other tree scopes.
+  assert_false(shadow2.firstChild instanceof TestCustomizedBuiltIn, 'tree scope without registry');
+  assert_false(shadow3.firstChild instanceof TestCustomizedBuiltIn, 'tree scope with different registry');
+  assert_false(testdiv.firstChild instanceof TestCustomizedBuiltIn, 'main document');
+}, 'Upgrade into customized built-in element when definition is added');
+</script>
diff --git a/testing/web-platform/tests/custom-elements/scoped-registry/constructor-call.tentative.html b/testing/web-platform/tests/custom-elements/scoped-registry/constructor-call.tentative.html
new file mode 100644
index 0000000000000..19a8e3f4d8ac6
--- /dev/null
+++ b/testing/web-platform/tests/custom-elements/scoped-registry/constructor-call.tentative.html
@@ -0,0 +1,42 @@
+<!DOCTYPE html>
+<meta name="author" title="Xiaocheng Hu" href="mailto:xiaochengh@chromium.org">
+<meta name="assert" content="Direct calls of custom element constructor use the global registry only">
+<link rel="help" href="https://wicg.github.io/webcomponents/proposals/Scoped-Custom-Element-Registries">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+
+<body>
+<script>
+function attachShadowForTest(t, registry) {
+  const host = document.createElement('div');
+  const shadow = host.attachShadow({mode: 'open', registry});
+  document.body.appendChild(host);
+  t.add_cleanup(() => host.remove());
+  return shadow;
+}
+
+test(t => {
+  class TestElement extends HTMLElement {};
+  let registry = new CustomElementRegistry()
+  registry.define('test-element', TestElement);
+
+  let shadow = attachShadowForTest(t, registry);
+
+  assert_throws_js(TypeError, () => new TestElement);
+}, 'Calling custom element constructor directly without global registration should fail');
+
+test(t => {
+  class TestElement extends HTMLElement {};
+
+  window.customElements.define('global-test-element', TestElement);
+
+  let registry = new CustomElementRegistry()
+  registry.define('shadow-test-element', TestElement);
+  let shadow = attachShadowForTest(t, registry);
+
+  let element = new TestElement;
+  assert_equals(element.localName, 'global-test-element');
+}, 'Calling custom element constructor directly uses global registration only');
+
+</script>
+</body>
diff --git a/testing/web-platform/tests/custom-elements/scoped-registry/constructor-reentry-with-different-definition.tentative.html b/testing/web-platform/tests/custom-elements/scoped-registry/constructor-reentry-with-different-definition.tentative.html
new file mode 100644
index 0000000000000..dc93e3c702d57
--- /dev/null
+++ b/testing/web-platform/tests/custom-elements/scoped-registry/constructor-reentry-with-different-definition.tentative.html
@@ -0,0 +1,137 @@
+<!DOCTYPE html>
+<meta name="author" title="Xiaocheng Hu" href="mailto:xiaochengh@chromium.org">
+<meta name="assert" content="Custom element constructors can re-enter with different definitions">
+<link rel="help" href="https://wicg.github.io/webcomponents/proposals/Scoped-Custom-Element-Registries">
+<link rel="help" href="https://github.com/WICG/webcomponents/issues/969">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+
+<div id='test-container-1'></div>
+<div id='test-container-2'></div>
+
+<script>
+setup({allow_uncaught_exception : true});
+
+function createShadowForTest(t, registry) {
+  const host = document.createElement('div');
+  const shadow = host.attachShadow({mode: 'open', registry});
+  document.body.appendChild(host);
+  t.add_cleanup(() => host.remove());
+  return shadow;
+}
+
+test(t => {
+  let needsTest = true;
+  class ReentryBeforeSuper extends HTMLElement {
+    constructor() {
+      if (needsTest) {
+        needsTest = false;
+        document.getElementById('test-container-1').innerHTML =
+            '<test-element-1></test-element-1>';
+      }
+      super();
+    }
+  };
+  window.customElements.define('test-element-1', ReentryBeforeSuper);
+
+  let registry = new CustomElementRegistry;
+  registry.define('shadow-test-element-1', ReentryBeforeSuper);
+
+  let shadow = createShadowForTest(t, registry);
+  shadow.innerHTML = '<shadow-test-element-1></shadow-test-element-1>';
+
+  let shadowElement = shadow.firstChild;
+  assert_true(shadowElement instanceof ReentryBeforeSuper);
+  assert_equals(shadowElement.localName, 'shadow-test-element-1');
+
+  let mainDocElement = document.getElementById('test-container-1').firstChild;
+  assert_true(mainDocElement instanceof ReentryBeforeSuper);
+  assert_equals(mainDocElement.localName, 'test-element-1');
+}, 'Re-entry via upgrade before calling super()');
+
+test(t => {
+  let needsTest = true;
+  class ReentryAfterSuper extends HTMLElement {
+    constructor() {
+      super();
+      if (needsTest) {
+        needsTest = false;
+        document.getElementById('test-container-2').innerHTML =
+            '<test-element-2></test-element-2>';
+      }
+    }
+  };
+  window.customElements.define('test-element-2', ReentryAfterSuper);
+
+  let registry = new CustomElementRegistry;
+  registry.define('shadow-test-element-2', ReentryAfterSuper);
+
+  let shadow = createShadowForTest(t, registry);
+  shadow.innerHTML = '<shadow-test-element-2></shadow-test-element-2>';
+
+  let shadowElement = shadow.firstChild;
+  assert_true(shadowElement instanceof ReentryAfterSuper);
+  assert_equals(shadowElement.localName, 'shadow-test-element-2');
+
+  let mainDocElement = document.getElementById('test-container-2').firstChild;
+  assert_true(mainDocElement instanceof ReentryAfterSuper);
+  assert_equals(mainDocElement.localName, 'test-element-2');
+}, 'Re-entry via upgrade after calling super()');
+
+test(t => {
+  let needsTest = true;
+  let elementByNestedCall;
+  class ReentryByDirectCall extends HTMLElement {
+    constructor() {
+      if (needsTest) {
+        needsTest = false;
+        elementByNestedCall = new ReentryByDirectCall;
+      }
+      super();
+    }
+  }
+  window.customElements.define('test-element-3', ReentryByDirectCall);
+
+  let registry = new CustomElementRegistry;
+  registry.define('shadow-test-element-3', ReentryByDirectCall);
+
+  let shadow = createShadowForTest(t, registry);
+  shadow.innerHTML = '<shadow-test-element-3></shadow-test-element-3>';
+
+  let shadowElement = shadow.firstChild;
+  assert_true(shadowElement instanceof ReentryByDirectCall);
+  assert_equals(shadowElement.localName, 'shadow-test-element-3');
+
+  // Nested constructor call makes the following `super()` fail, and we should
+  // end up creating only one element.
+  assert_equals(elementByNestedCall, shadowElement);
+}, 'Re-entry via direct constructor call before calling super()');
+
+test(t => {
+  let needsTest = true;
+  let elementByNestedCall;
+  class ReentryByDirectCall extends HTMLElement {
+    constructor() {
+      super();
+      if (needsTest) {
+        needsTest = false;
+        elementByNestedCall = new ReentryByDirectCall;
+      }
+    }
+  }
+  window.customElements.define('test-element-4', ReentryByDirectCall);
+
+  let registry = new CustomElementRegistry;
+  registry.define('shadow-test-element-4', ReentryByDirectCall);
+
+  let shadow = createShadowForTest(t, registry);
+  shadow.innerHTML = '<shadow-test-element-4></shadow-test-element-4>';
+
+  let shadowElement = shadow.firstChild;
+  assert_true(shadowElement instanceof ReentryByDirectCall);
+  assert_equals(shadowElement.localName, 'shadow-test-element-4');
+
+  // Nested constructor call should be blocked.
+  assert_false(elementByNestedCall instanceof ReentryByDirectCall);
+}, 'Re-entry via direct constructor call after calling super()');
+</script>
