diff --git a/testing/web-platform/tests/shadow-dom/declarative/declarative-shadow-dom-attachment.html b/testing/web-platform/tests/shadow-dom/declarative/declarative-shadow-dom-attachment.html
index 6fb2a8a5ecc30..aec9a276f931f 100644
--- a/testing/web-platform/tests/shadow-dom/declarative/declarative-shadow-dom-attachment.html
+++ b/testing/web-platform/tests/shadow-dom/declarative/declarative-shadow-dom-attachment.html
@@ -42,9 +42,13 @@
         originalShadowRoot = element.shadowRoot;
       }
 
-      // Now, call attachShadow() and make sure we get back the same (original) shadowRoot, but empty.
       const oppositeMode = (mode === 'open') ? 'closed' : 'open';
-      const newShadow = element.attachShadow({mode: oppositeMode}); // Should be no exception here
+      assert_throws_dom('NotSupportedError', () => {
+        element.attachShadow({mode: oppositeMode});
+      }, 'Calling attachShadow with a declarative shadow fails if the mode doesn\'t match');
+
+      // Now, call attachShadow() and make sure we get back the same (original) shadowRoot, but empty.
+      const newShadow = element.attachShadow({mode: mode, delegatesFocus: delegatesFocus});
       if (mode === 'open') {
         assert_equals(element.shadowRoot, originalShadowRoot, 'The same shadow root should be returned');
         assert_equals(element.shadowRoot.innerHTML, '', 'Empty shadow content');
diff --git a/testing/web-platform/tests/shadow-dom/declarative/declarative-shadow-dom-basic.html b/testing/web-platform/tests/shadow-dom/declarative/declarative-shadow-dom-basic.html
index 20ca34f6f3e25..8799978804a53 100644
--- a/testing/web-platform/tests/shadow-dom/declarative/declarative-shadow-dom-basic.html
+++ b/testing/web-platform/tests/shadow-dom/declarative/declarative-shadow-dom-basic.html
@@ -154,10 +154,14 @@
 <script>
 test(() => {
   const host = document.querySelector('#multi-host');
-  assert_equals(host.querySelector('template'), null, "No leftover template nodes from either root");
+  const leftover = host.querySelector('template');
+  assert_true(!!leftover, "The second (duplicate) template should be left in the DOM");
+  assert_true(leftover instanceof HTMLTemplateElement);
+  assert_equals(leftover.getAttribute('shadowrootmode'),"closed");
+  assert_equals(leftover.shadowRootMode,"closed");
   assert_true(!!host.shadowRoot,"No open shadow root found - first root should remain");
   const innerSpan = host.shadowRoot.querySelector('span');
-  assert_equals(innerSpan.textContent, 'root 2', "Content should come from last declarative shadow root");
+  assert_equals(innerSpan.textContent, 'root 1', "Content should come from first declarative shadow root");
 }, 'Declarative Shadow DOM: Multiple roots');
 
 </script>
diff --git a/testing/web-platform/tests/shadow-dom/declarative/declarative-shadow-dom-repeats.html b/testing/web-platform/tests/shadow-dom/declarative/declarative-shadow-dom-repeats.html
new file mode 100644
index 0000000000000..3da9cff360c64
--- /dev/null
+++ b/testing/web-platform/tests/shadow-dom/declarative/declarative-shadow-dom-repeats.html
@@ -0,0 +1,73 @@
+<!DOCTYPE html>
+<title>Declarative Shadow DOM Element Attachment</title>
+<link rel='author' href='mailto:masonf@chromium.org'>
+<link rel='help' href='https://github.com/whatwg/dom/issues/1235'>
+<link rel='help' href='https://github.com/whatwg/html/pull/10069'>
+<link rel='help' href='https://github.com/whatwg/dom/pull/1246'>
+
+<script src='/resources/testharness.js'></script>
+<script src='/resources/testharnessreport.js'></script>
+<script src='../../html/resources/common.js'></script>
+<script src="support/helpers.js"></script>
+
+<div id=multiple1>
+  <template shadowrootmode=open>Open</template>
+  <template shadowrootmode=closed>Closed</template>
+</div>
+
+<div id=multiple2>
+  <template shadowrootmode=closed>Closed</template>
+  <template shadowrootmode=open>Open</template>
+</div>
+
+<script>
+test((t) => {
+  let shadow = multiple1.shadowRoot;
+  assert_true(!!shadow,'Remaining shadow root should be open');
+  assert_equals(shadow.textContent,"Open");
+  shadow = multiple2.shadowRoot;
+  assert_false(!!shadow,'Remaining shadow root should be closed');
+  multiple1.remove(); // Cleanup
+  multiple2.remove();
+},'Repeated declarative shadow roots keep only the first');
+</script>
+
+<div id=open1>
+  <template shadowrootmode=open>Open</template>
+</div>
+
+<script>
+test((t) => {
+  assert_throws_dom("NotSupportedError",() => {
+    open1.attachShadow({mode: "closed"});
+  },'Mismatched shadow root type should throw');
+  const initialShadow = open1.shadowRoot;
+  const shadow = open1.attachShadow({mode: "open"}); // Shouldn't throw
+  assert_equals(shadow,initialShadow,'Same shadow should be returned');
+  assert_equals(shadow.textContent,'','Shadow should be empty');
+},'Calling attachShadow() on declarative shadow root must match type');
+</script>
+
+<div id=open2>
+  <template shadowrootmode=open shadowrootdelegatesfocus>
+    Open, delegates focus (not the default), named slot assignment (the default)
+  </template>
+</div>
+
+<script>
+test((t) => {
+  assert_throws_dom("NotSupportedError",() => {
+    open2.attachShadow({mode: "closed", delegatesFocus: true, slotAssignment: "named"});
+  },'Mismatched shadow root type should throw');
+  assert_throws_dom("NotSupportedError",() => {
+    open2.attachShadow({mode: "open", delegatesFocus: false, slotAssignment: "named"});
+  },'Mismatched shadow root delegatesFocus should throw');
+  assert_throws_dom("NotSupportedError",() => {
+    open2.attachShadow({mode: "open", delegatesFocus: true, slotAssignment: "manual"});
+  },'Mismatched shadow root slotAssignment should throw');
+  const initialShadow = open2.shadowRoot;
+  const shadow = open2.attachShadow({mode: "open", delegatesFocus: true, slotAssignment: "named"}); // Shouldn't throw
+  assert_equals(shadow,initialShadow,'Same shadow should be returned');
+  assert_equals(shadow.textContent,'','Shadow should be empty');
+},'Calling attachShadow() on declarative shadow root must match all parameters');
+</script>
