diff --git a/packages/happy-dom/src/nodes/html-template-element/HTMLTemplateElement.ts b/packages/happy-dom/src/nodes/html-template-element/HTMLTemplateElement.ts
index 8911a01da..4b99ed0a5 100644
--- a/packages/happy-dom/src/nodes/html-template-element/HTMLTemplateElement.ts
+++ b/packages/happy-dom/src/nodes/html-template-element/HTMLTemplateElement.ts
@@ -1,11 +1,9 @@
-import Node from '../node/Node';
 import HTMLElement from '../html-element/HTMLElement';
 import IDocumentFragment from '../document-fragment/IDocumentFragment';
 import INode from '../node/INode';
 import IHTMLTemplateElement from './IHTMLTemplateElement';
 import XMLParser from '../../xml-parser/XMLParser';
 import XMLSerializer from '../../xml-serializer/XMLSerializer';
-import DOMException from '../../exception/DOMException';
 
 /**
  * HTML Template Element.
@@ -36,34 +34,18 @@ export default class HTMLTemplateElement extends HTMLElement implements IHTMLTem
 		}
 	}
 
-	/**
-	 * @override
-	 */
-	public get outerHTML(): string {
-		return new XMLSerializer().serializeToString(this.content);
-	}
-
-	/**
-	 * @override
-	 */
-	public set outerHTML(_html: string) {
-		throw new DOMException(
-			`Failed to set the 'outerHTML' property on 'Element': This element has no parent node.`
-		);
-	}
-
 	/**
 	 * @override
 	 */
 	public get previousSibling(): INode {
-		return this.content.previousSibling;
+		return null;
 	}
 
 	/**
 	 * @override
 	 */
 	public get nextSibling(): INode {
-		return this.content.nextSibling;
+		return null;
 	}
 
 	/**
@@ -102,7 +84,7 @@ export default class HTMLTemplateElement extends HTMLElement implements IHTMLTem
 	/**
 	 * @override
 	 */
-	public removeChild(node: Node): INode {
+	public removeChild(node: INode): INode {
 		return this.content.removeChild(node);
 	}
 
diff --git a/packages/happy-dom/src/xml-serializer/XMLSerializer.ts b/packages/happy-dom/src/xml-serializer/XMLSerializer.ts
index 00814fcd7..9a255bb57 100644
--- a/packages/happy-dom/src/xml-serializer/XMLSerializer.ts
+++ b/packages/happy-dom/src/xml-serializer/XMLSerializer.ts
@@ -5,6 +5,7 @@ import DocumentType from '../nodes/document-type/DocumentType';
 import { escape } from 'he';
 import INode from '../nodes/node/INode';
 import IElement from '../nodes/element/IElement';
+import IHTMLTemplateElement from '../nodes/html-template-element/IHTMLTemplateElement';
 
 /**
  * Utility for converting an element to string.
@@ -28,9 +29,13 @@ export default class XMLSerializer {
 					return `<${tagName}${this._getAttributes(element)}>`;
 				}
 
+				const childNodes =
+					element.tagName === 'TEMPLATE'
+						? (<IHTMLTemplateElement>root).content.childNodes
+						: root.childNodes;
 				let innerHTML = '';
 
-				for (const node of root.childNodes) {
+				for (const node of childNodes) {
 					innerHTML += this.serializeToString(node, options);
 				}
 
diff --git a/packages/happy-dom/test/nodes/html-template-element/HTMLTemplateElement.test.ts b/packages/happy-dom/test/nodes/html-template-element/HTMLTemplateElement.test.ts
index 22d3879c3..4fcd54d7c 100644
--- a/packages/happy-dom/test/nodes/html-template-element/HTMLTemplateElement.test.ts
+++ b/packages/happy-dom/test/nodes/html-template-element/HTMLTemplateElement.test.ts
@@ -1,6 +1,7 @@
 import Window from '../../../src/window/Window';
 import Document from '../../../src/nodes/document/Document';
 import HTMLTemplateElement from '../../../src/nodes/html-template-element/HTMLTemplateElement';
+import XMLSerializer from '../../../src/xml-serializer/XMLSerializer';
 
 describe('HTMLTemplateElement', () => {
 	let window: Window;
@@ -17,13 +18,215 @@ describe('HTMLTemplateElement', () => {
 		jest.restoreAllMocks();
 	});
 
-	it('InnerHTML', () => {
-		const div = '<div>happy-dom is cool!</div>';
-		expect(element.content.childNodes.length).toBe(0);
-		element.innerHTML = div;
-		expect(element.innerHTML).toBe(div);
-		expect(element.content.childNodes.length).toBe(1);
-		element.innerHTML = '';
-		expect(element.content.childNodes.length).toBe(0);
+	describe('get innerHTML()', () => {
+		it('Returns inner HTML of the "content" node.', () => {
+			const div = document.createElement('div');
+
+			div.innerHTML = 'Test';
+
+			expect(element.content.childNodes.length).toBe(0);
+			expect(element.innerHTML).toBe('');
+
+			element.appendChild(div);
+
+			expect(element.childNodes.length).toBe(0);
+			expect(element.innerHTML).toBe('<div>Test</div>');
+			expect(new XMLSerializer().serializeToString(element.content)).toBe('<div>Test</div>');
+
+			element.removeChild(div);
+
+			expect(element.content.childNodes.length).toBe(0);
+			expect(element.innerHTML).toBe('');
+		});
+	});
+
+	describe('set innerHTML()', () => {
+		it('Serializes the HTML into nodes and appends them to the "content" node.', () => {
+			expect(element.content.childNodes.length).toBe(0);
+			expect(element.innerHTML).toBe('');
+
+			element.innerHTML = '<div>Test</div>';
+
+			expect(element.childNodes.length).toBe(0);
+			expect(element.innerHTML).toBe('<div>Test</div>');
+			expect(new XMLSerializer().serializeToString(element.content)).toBe('<div>Test</div>');
+
+			element.innerHTML = '';
+
+			expect(element.content.childNodes.length).toBe(0);
+			expect(element.innerHTML).toBe('');
+		});
+	});
+
+	describe('get outerHTML()', () => {
+		it('Serializes the HTML into nodes and appends them to the "content" node.', () => {
+			expect(element.content.childNodes.length).toBe(0);
+			expect(element.innerHTML).toBe('');
+
+			element.innerHTML = '<div>Test</div>';
+
+			expect(element.childNodes.length).toBe(0);
+			expect(element.outerHTML).toBe('<template><div>Test</div></template>');
+
+			element.innerHTML = '';
+
+			expect(element.outerHTML).toBe('<template></template>');
+		});
+	});
+
+	describe('set outerHTML()', () => {
+		it('Replaces the template with a span.', () => {
+			element.innerHTML = '<div>Test</div>';
+
+			document.body.appendChild(element);
+
+			expect(document.body.innerHTML).toBe('<template><div>Test</div></template>');
+
+			element.outerHTML = '<span>Test</span>';
+
+			expect(document.body.innerHTML).toBe('<span>Test</span>');
+		});
+	});
+
+	describe('get previousSibling()', () => {
+		it('Returns null.', () => {
+			element.innerHTML = '<div>Test</div>';
+			expect(element.previousSibling).toBe(null);
+		});
+	});
+
+	describe('get nextSibling()', () => {
+		it('Returns null.', () => {
+			element.innerHTML = '<div>Test</div>';
+			expect(element.nextSibling).toBe(null);
+		});
+	});
+
+	describe('get firstChild()', () => {
+		it('Returns first child.', () => {
+			const div = document.createElement('div');
+			const span = document.createElement('span');
+			element.appendChild(div);
+			element.appendChild(span);
+			expect(element.firstChild).toBe(div);
+		});
+	});
+
+	describe('get lastChild()', () => {
+		it('Returns last child.', () => {
+			const div = document.createElement('div');
+			const span = document.createElement('span');
+			element.appendChild(div);
+			element.appendChild(span);
+			expect(element.lastChild).toBe(span);
+		});
+	});
+
+	describe('getInnerHTML()', () => {
+		it('Returns inner HTML of the "content" node.', () => {
+			const div = document.createElement('div');
+
+			div.innerHTML = 'Test';
+
+			expect(element.content.childNodes.length).toBe(0);
+			expect(element.getInnerHTML()).toBe('');
+
+			element.appendChild(div);
+
+			expect(element.childNodes.length).toBe(0);
+			expect(element.getInnerHTML()).toBe('<div>Test</div>');
+			expect(new XMLSerializer().serializeToString(element.content)).toBe('<div>Test</div>');
+
+			element.removeChild(div);
+
+			expect(element.content.childNodes.length).toBe(0);
+			expect(element.getInnerHTML()).toBe('');
+		});
+
+		it('Returns HTML of children and shadow roots of custom elements as a concatenated string.', () => {
+			const div = document.createElement('div');
+
+			element.appendChild(div);
+
+			jest
+				.spyOn(XMLSerializer.prototype, 'serializeToString')
+				.mockImplementation((rootElement, options) => {
+					expect(rootElement).toBe(div);
+					expect(options).toEqual({ includeShadowRoots: true });
+					return 'EXPECTED_HTML';
+				});
+
+			expect(element.getInnerHTML({ includeShadowRoots: true })).toBe('EXPECTED_HTML');
+		});
+	});
+
+	describe('appendChild()', () => {
+		it('Appends a node to the "content" node.', () => {
+			const div = document.createElement('div');
+
+			expect(element.childNodes.length).toBe(0);
+			expect(element.content.childNodes.length).toBe(0);
+
+			element.appendChild(div);
+
+			expect(element.childNodes.length).toBe(0);
+			expect(element.content.childNodes.length).toBe(1);
+			expect(element.content.childNodes[0] === div).toBe(true);
+
+			element.removeChild(div);
+
+			expect(element.childNodes.length).toBe(0);
+			expect(element.content.childNodes.length).toBe(0);
+		});
+	});
+
+	describe('removeChild()', () => {
+		it('Removes a node from the "content" node.', () => {
+			const div = document.createElement('div');
+
+			element.appendChild(div);
+
+			expect(element.childNodes.length).toBe(0);
+			expect(element.content.childNodes.length).toBe(1);
+
+			element.removeChild(div);
+
+			expect(element.childNodes.length).toBe(0);
+			expect(element.content.childNodes.length).toBe(0);
+		});
+	});
+
+	describe('insertBefore()', () => {
+		it('Inserts a node before another node in the "content" node.', () => {
+			const div = document.createElement('div');
+			const span = document.createElement('span');
+			const underline = document.createElement('u');
+			element.appendChild(div);
+			element.appendChild(span);
+			element.insertBefore(underline, span);
+			expect(element.innerHTML).toBe('<div></div><u></u><span></span>');
+		});
+	});
+
+	describe('replaceChild()', () => {
+		it('Removes a node from the "content" node.', () => {
+			const div = document.createElement('div');
+			const span = document.createElement('span');
+			const underline = document.createElement('u');
+			const bold = document.createElement('b');
+			element.appendChild(div);
+			element.appendChild(underline);
+			element.appendChild(span);
+			element.replaceChild(bold, underline);
+			expect(element.innerHTML).toBe('<div></div><b></b><span></span>');
+		});
+	});
+
+	describe('cloneNode()', () => {
+		it('Clones the nodes of the "content" node.', () => {
+			element.innerHTML = '<div></div><b></b><span></span>';
+			const clone = element.cloneNode(true);
+			expect(clone.innerHTML).toBe('<div></div><b></b><span></span>');
+		});
 	});
 });
diff --git a/packages/happy-dom/test/xml-parser/XMLParser.test.ts b/packages/happy-dom/test/xml-parser/XMLParser.test.ts
index 1909ffef9..c2ef811a8 100644
--- a/packages/happy-dom/test/xml-parser/XMLParser.test.ts
+++ b/packages/happy-dom/test/xml-parser/XMLParser.test.ts
@@ -167,7 +167,7 @@ describe('XMLParser', () => {
 			const root = XMLParser.parse(
 				window.document,
 				`<div>
-					<script>if(1<Math['random']()){else if(Math['random']()>1){console.log("1")}</script>
+					<script>if(1<Math['random']()){}else if(Math['random']()>1){console.log("1")}</script>
 					<script><b></b></script>
 					<style><b></b></style>
 					<template><b></b></template>
@@ -175,7 +175,7 @@ describe('XMLParser', () => {
 			);
 
 			expect((<IHTMLElement>root.children[0].children[0]).innerText).toBe(
-				`if(1<Math['random']()){else if(Math['random']()>1){console.log("1")}`
+				`if(1<Math['random']()){}else if(Math['random']()>1){console.log("1")}`
 			);
 
 			expect((<IHTMLElement>root.children[0].children[1]).innerText).toBe('<b></b>');
@@ -186,10 +186,10 @@ describe('XMLParser', () => {
 
 			expect(new XMLSerializer().serializeToString(root).replace(/[\s]/gm, '')).toBe(
 				`<div>
-					<script>if(1<Math['random']()){else if(Math['random']()>1){console.log("1")}</script>
+					<script>if(1<Math['random']()){}else if(Math['random']()>1){console.log("1")}</script>
 					<script><b></b></script>
 					<style><b></b></style>
-					<template></template>
+					<template><b></b></template>
 				</div>`.replace(/[\s]/gm, '')
 			);
 
diff --git a/packages/happy-dom/test/xml-serializer/XMLSerializer.test.ts b/packages/happy-dom/test/xml-serializer/XMLSerializer.test.ts
index d50c15511..fe12593b8 100644
--- a/packages/happy-dom/test/xml-serializer/XMLSerializer.test.ts
+++ b/packages/happy-dom/test/xml-serializer/XMLSerializer.test.ts
@@ -74,6 +74,14 @@ describe('XMLSerializer', () => {
 			expect(xmlSerializer.serializeToString(div)).toBe('<div>Text 1.Text 2.</div>');
 		});
 
+		it('Serializes a template node.', () => {
+			const template = document.createElement('template');
+			template.innerHTML = '<div>Test</div>';
+			expect(xmlSerializer.serializeToString(template)).toBe(
+				'<template><div>Test</div></template>'
+			);
+		});
+
 		it('Serializes a mix of nodes.', () => {
 			const div = document.createElement('div');
 			const comment1 = document.createComment('Comment 1.');