diff --git a/src/mock-doc/serialize-node.ts b/src/mock-doc/serialize-node.ts
index 75e6d5726fa..905fce9aa84 100644
--- a/src/mock-doc/serialize-node.ts
+++ b/src/mock-doc/serialize-node.ts
@@ -132,7 +132,11 @@ function* streamToHtml(
        */
       if (
         tag === 'template' &&
-        (!(node as Element).getAttribute || !(node as Element).getAttribute('shadowrootmode'))
+        (!(node as Element).getAttribute || !(node as Element).getAttribute('shadowrootmode')) &&
+        /**
+         * If the node is a shadow root, we want to add the `shadowrootmode` attribute
+         */
+        ('host' in node || node.nodeName.toLocaleLowerCase() === shadowRootTag)
       ) {
         const mode = ` shadowrootmode="open"`;
         yield mode;
diff --git a/src/mock-doc/test/html-parse.spec.ts b/src/mock-doc/test/html-parse.spec.ts
index 2ab979698f3..d42a5610efd 100644
--- a/src/mock-doc/test/html-parse.spec.ts
+++ b/src/mock-doc/test/html-parse.spec.ts
@@ -127,11 +127,11 @@ describe('parseHtml', () => {
       <template>text</template>
     `);
 
-    expect(doc.head.innerHTML).toBe(`<template shadowrootmode="open">text</template>`);
+    expect(doc.head.innerHTML).toBe(`<template>text</template>`);
 
     const tmplElm: HTMLTemplateElement = doc.head.firstElementChild as any;
 
-    expect(tmplElm.outerHTML).toBe(`<template shadowrootmode="open">text</template>`);
+    expect(tmplElm.outerHTML).toBe(`<template>text</template>`);
     expect(tmplElm.content?.firstChild?.textContent).toBe(`text`);
     expect(tmplElm.childNodes).toHaveLength(0);
   });
diff --git a/src/mock-doc/test/serialize-node.spec.ts b/src/mock-doc/test/serialize-node.spec.ts
index a7da936eb16..961990a5573 100644
--- a/src/mock-doc/test/serialize-node.spec.ts
+++ b/src/mock-doc/test/serialize-node.spec.ts
@@ -166,7 +166,7 @@ describe('serializeNodeToHtml', () => {
     doc.body.innerHTML = input;
 
     const output = serializeNodeToHtml(doc.body);
-    expect(output).toBe(`<template shadowrootmode="open">text</template>`);
+    expect(output).toBe(`<template>text</template>`);
   });
 
   it('svg', () => {