diff --git a/shadow-dom/declarative/gethtml.tentative.html b/shadow-dom/declarative/gethtml.tentative.html
new file mode 100644
index 000000000000000..3a7b627b09fa4e5
--- /dev/null
+++ b/shadow-dom/declarative/gethtml.tentative.html
@@ -0,0 +1,101 @@
+<!DOCTYPE html>
+<title>getHTML </title>
+<link rel='author' href='mailto:masonf@chromium.org'>
+<link rel='help' href='https://github.com/whatwg/html/issues/8867'>
+<script src='/resources/testharness.js'></script>
+<script src='/resources/testharnessreport.js'></script>
+<script src='../../html/resources/common.js'></script>
+
+<body>
+
+<script>
+function testElementType(allowsShadowDom, elementType, runGetHTMLOnShadowRoot, mode, delegatesFocus, serializable) {
+  const t = test(t => {
+    // Create and attach element
+    let wrapper;
+    if (runGetHTMLOnShadowRoot) {
+      // This ensures we're testing both Element.getHTML() and ShadowRoot.getHTML().
+      const host = document.createElement('div');
+      t.add_cleanup(function() { host.remove(); });
+      document.body.appendChild(host);
+      wrapper = host.attachShadow({mode: 'open'});
+    } else {
+      wrapper = document.createElement('div');
+      t.add_cleanup(function() { wrapper.remove(); });
+      document.body.appendChild(wrapper);
+    }
+    const element = document.createElement(elementType);
+    wrapper.appendChild(element);
+
+    const isOpen = mode === 'open';
+    if (allowsShadowDom) {
+      const delegatesAttr = delegatesFocus ? ' shadowrootdelegatesfocus=""' : '';
+      const correctShadowHtml = `<template shadowrootmode="${mode}"${delegatesAttr}><slot></slot></template>`;
+      const correctHtml = `<${elementType}>${correctShadowHtml}</${elementType}>`;
+      let initDict = {mode: mode, delegatesFocus: delegatesFocus};
+      let expectedSerializable = null;
+      switch (serializable) {
+        case "none": expectedSerializable = false; break;
+        case "true": initDict.serializable = expectedSerializable = true; break;
+        case "false": initDict.serializable = expectedSerializable = false; break;
+        default: throw new Error("Invalid");
+      }
+      const shadowRoot = element.attachShadow(initDict);
+      assert_equals(shadowRoot.mode,mode);
+      assert_equals(shadowRoot.delegatesFocus,delegatesFocus);
+      assert_equals(shadowRoot.serializable,expectedSerializable);
+      shadowRoot.appendChild(document.createElement('slot'));
+      const emptyElement = `<${elementType}></${elementType}>`;
+      if (isOpen) {
+        if (expectedSerializable) {
+          assert_equals(wrapper.getHTML({includeShadowRoots: true}), correctHtml);
+        } else {
+          assert_equals(wrapper.getHTML({includeShadowRoots: true}), emptyElement);
+        }
+      } else {
+        // Closed shadow roots should not be returned unless shadowRoots specifically contains the shadow root:
+        assert_equals(wrapper.getHTML({includeShadowRoots: true}), emptyElement);
+        assert_equals(wrapper.getHTML({includeShadowRoots: true, shadowRoots: []}), emptyElement);
+      }
+      // If we provide the shadow root, serialize it, regardless of includeShadowRoots.
+      assert_equals(wrapper.getHTML({includeShadowRoots: true, shadowRoots: [shadowRoot]}),correctHtml);
+      assert_equals(wrapper.getHTML({shadowRoots: [shadowRoot]}),correctHtml);
+      // This should always throw - includeShadowRoots false, but we've provided roots.
+      assert_throws_dom("NotSupportedError",() => wrapper.getHTML({includeShadowRoots: false, shadowRoots: [shadowRoot]}));
+    } else {
+      // For non-shadow hosts, getHTML() should also match .innerHTML
+      assert_equals(wrapper.getHTML({includeShadowRoots: true}),wrapper.innerHTML);
+    }
+
+    // Either way, make sure getHTML({includeShadowRoots: false}) matches .innerHTML
+    assert_equals(wrapper.getHTML({includeShadowRoots: false}),wrapper.innerHTML,'getHTML() with includeShadowRoots false should return the same as .innerHTML');
+    // ...and that the default for includeShadowRoots is false.
+    assert_equals(wrapper.getHTML(),wrapper.innerHTML,'The default for includeShadowRoots should be false');
+
+  }, `${runGetHTMLOnShadowRoot ? 'ShadowRoot' : 'Element'}.getHTML() on <${elementType}>${allowsShadowDom ? `, with mode=${mode}, delegatesFocus=${delegatesFocus}, serializable=${serializable}.` : ''}`);
+}
+
+function runAllTests() {
+  const allElements = [...HTML5_ELEMENTS, 'htmlunknown'];
+  const safelisted = HTML5_SHADOW_ALLOWED_ELEMENTS;
+  for (const elementName of allElements) {
+    const allowsShadowDom = safelisted.includes(elementName);
+    for (const runGetHTMLOnShadowRoot of [false, true]) {
+      if (allowsShadowDom) {
+        for (const delegatesFocus of [false, true]) {
+          for (const mode of ['open', 'closed']) {
+            for (const serializable of ['none', 'false', 'true']) {
+              testElementType(true, elementName, runGetHTMLOnShadowRoot, mode, delegatesFocus, serializable);
+            }
+          }
+        }
+      } else {
+        testElementType(false, elementName, runGetHTMLOnShadowRoot);
+      }
+    }
+  }
+}
+
+runAllTests();
+
+</script>
