diff --git a/packages/happy-dom/src/html-serializer/HTMLSerializer.ts b/packages/happy-dom/src/html-serializer/HTMLSerializer.ts
index 58b90d4dd..61bd1cb83 100644
--- a/packages/happy-dom/src/html-serializer/HTMLSerializer.ts
+++ b/packages/happy-dom/src/html-serializer/HTMLSerializer.ts
@@ -15,7 +15,7 @@ import HTMLElementConfigContentModelEnum from '../config/HTMLElementConfigConten
  * Serializes a node into HTML.
  */
 export default class HTMLSerializer {
-	public [PropertySymbol.options]: {
+	private options: {
 		serializableShadowRoots: boolean;
 		shadowRoots: ShadowRoot[] | null;
 		allShadowRoots: boolean;
@@ -25,6 +25,34 @@ export default class HTMLSerializer {
 		allShadowRoots: false
 	};
 
+	/**
+	 * Constructor.
+	 *
+	 * @param [options] Options.
+	 * @param [options.serializableShadowRoots] If shadow roots should be serialized.
+	 * @param [options.shadowRoots] Shadow roots to serialize.
+	 * @param [options.allShadowRoots] If all shadow roots should be serialized.
+	 */
+	constructor(options?: {
+		serializableShadowRoots?: boolean;
+		shadowRoots?: ShadowRoot[] | null;
+		allShadowRoots?: boolean;
+	}) {
+		if (options) {
+			if (options.serializableShadowRoots) {
+				this.options.serializableShadowRoots = options.serializableShadowRoots;
+			}
+
+			if (options.shadowRoots) {
+				this.options.shadowRoots = options.shadowRoots;
+			}
+
+			if (options.allShadowRoots) {
+				this.options.allShadowRoots = options.allShadowRoots;
+			}
+		}
+	}
+
 	/**
 	 * Renders an element as HTML.
 	 *
@@ -32,8 +60,6 @@ export default class HTMLSerializer {
 	 * @returns Result.
 	 */
 	public serializeToString(root: Node): string {
-		const options = this[PropertySymbol.options];
-
 		switch (root[PropertySymbol.nodeType]) {
 			case NodeTypeEnum.elementNode:
 				const element = <Element>root;
@@ -49,9 +75,10 @@ export default class HTMLSerializer {
 				// TODO: Should we include closed shadow roots? We are currently only including open shadow roots.
 				if (
 					element.shadowRoot &&
-					(options.allShadowRoots ||
-						(options.serializableShadowRoots && element.shadowRoot[PropertySymbol.serializable]) ||
-						options.shadowRoots?.includes(element.shadowRoot))
+					(this.options.allShadowRoots ||
+						(this.options.serializableShadowRoots &&
+							element.shadowRoot[PropertySymbol.serializable]) ||
+						this.options.shadowRoots?.includes(element.shadowRoot))
 				) {
 					innerHTML += `<template shadowrootmode="${element.shadowRoot[PropertySymbol.mode]}"${
 						element.shadowRoot[PropertySymbol.serializable] ? ' shadowrootserializable=""' : ''
@@ -73,14 +100,6 @@ export default class HTMLSerializer {
 					innerHTML += this.serializeToString(node);
 				}
 
-				// if (
-				// 	!innerHTML &&
-				// 	(root[PropertySymbol.namespaceURI] === NamespaceURI.xmlns ||
-				// 		root[PropertySymbol.namespaceURI] === NamespaceURI.svg)
-				// ) {
-				// 	return `<${localName}${this.getAttributes(element)}/>`;
-				// }
-
 				return `<${localName}${this.getAttributes(element)}>${innerHTML}</${localName}>`;
 			case Node.DOCUMENT_FRAGMENT_NODE:
 			case Node.DOCUMENT_NODE:
@@ -92,7 +111,6 @@ export default class HTMLSerializer {
 			case NodeTypeEnum.commentNode:
 				return `<!--${root.textContent}-->`;
 			case NodeTypeEnum.processingInstructionNode:
-				// TODO: Add support for processing instructions.
 				return `<!--?${(<ProcessingInstruction>root).target} ${root.textContent}?-->`;
 			case NodeTypeEnum.textNode:
 				const parentElement = root.parentElement;
@@ -123,36 +141,15 @@ export default class HTMLSerializer {
 	private getAttributes(element: Element): string {
 		let attributeString = '';
 
-		if (
-			!(<Element>element)[PropertySymbol.attributes].getNamedItem('is') &&
-			(<Element>element)[PropertySymbol.isValue]
-		) {
-			attributeString += ' is="' + (<Element>element)[PropertySymbol.isValue] + '"';
-		}
-
 		const namedItems = (<Element>element)[PropertySymbol.attributes][PropertySymbol.namedItems];
 
-		// if (
-		// 	element[PropertySymbol.namespaceURI] === NamespaceURI.svg ||
-		// 	element[PropertySymbol.namespaceURI] === NamespaceURI.xmlns
-		// ) {
-		// 	const xmlns = namedItems.get('xmlns');
-
-		// 	// The "xmlns" attribute should always be the first attribute if it exists.
-		// 	if (xmlns && xmlns[PropertySymbol.value]) {
-		// 		attributeString += ' xmlns="' + xmlns[PropertySymbol.value] + '"';
-		// 	}
-		// }
+		if (!namedItems.has('is') && element[PropertySymbol.isValue]) {
+			attributeString += ' is="' + element[PropertySymbol.isValue] + '"';
+		}
 
 		for (const attribute of namedItems.values()) {
-			// if (
-			// 	attribute[PropertySymbol.name] !== 'xmlns' ||
-			// 	(element[PropertySymbol.namespaceURI] !== NamespaceURI.svg &&
-			// 		element[PropertySymbol.namespaceURI] !== NamespaceURI.xmlns)
-			// ) {
 			const escapedValue = Entities.escapeAttribute(attribute[PropertySymbol.value]);
 			attributeString += ' ' + attribute[PropertySymbol.name] + '="' + escapedValue + '"';
-			// }
 		}
 
 		return attributeString;
diff --git a/packages/happy-dom/src/nodes/element/Element.ts b/packages/happy-dom/src/nodes/element/Element.ts
index d1935fa46..c68fe8c26 100644
--- a/packages/happy-dom/src/nodes/element/Element.ts
+++ b/packages/happy-dom/src/nodes/element/Element.ts
@@ -5,7 +5,6 @@ import DOMRect from '../../dom/DOMRect.js';
 import DOMTokenList from '../../dom/DOMTokenList.js';
 import QuerySelector from '../../query-selector/QuerySelector.js';
 import XMLParser from '../../xml-parser/XMLParser.js';
-import XMLSerializer from '../../xml-serializer/XMLSerializer.js';
 import ChildNodeUtility from '../child-node/ChildNodeUtility.js';
 import ParentNodeUtility from '../parent-node/ParentNodeUtility.js';
 import NonDocumentChildNodeUtility from '../child-node/NonDocumentChildNodeUtility.js';
@@ -32,7 +31,7 @@ import NodeList from '../node/NodeList.js';
 import CSSStyleDeclaration from '../../css/declaration/CSSStyleDeclaration.js';
 import NamedNodeMapProxyFactory from './NamedNodeMapProxyFactory.js';
 import NodeFactory from '../NodeFactory.js';
-import XMLParserModeEnum from '../../xml-parser/XMLParserModeEnum.js';
+import HTMLSerializer from '../../html-serializer/HTMLSerializer.js';
 
 type InsertAdjacentPosition = 'beforebegin' | 'afterbegin' | 'beforeend' | 'afterend';
 
@@ -416,7 +415,7 @@ export default class Element
 	 * @returns HTML.
 	 */
 	public get outerHTML(): string {
-		return new XMLSerializer().serializeToString(this);
+		return new HTMLSerializer().serializeToString(this);
 	}
 
 	/**
@@ -495,19 +494,17 @@ export default class Element
 	 * @returns HTML.
 	 */
 	public getInnerHTML(options?: { includeShadowRoots?: boolean }): string {
-		const xmlSerializer = new XMLSerializer();
+		const serializer = new HTMLSerializer({
+			allShadowRoots: !!options?.includeShadowRoots
+		});
 
-		if (options?.includeShadowRoots) {
-			xmlSerializer[PropertySymbol.options].allShadowRoots = true;
-		}
-
-		let xml = '';
+		let html = '';
 
 		for (const node of this[PropertySymbol.nodeArray]) {
-			xml += xmlSerializer.serializeToString(node);
+			html += serializer.serializeToString(node);
 		}
 
-		return xml;
+		return html;
 	}
 
 	/**
@@ -522,25 +519,18 @@ export default class Element
 		serializableShadowRoots?: boolean;
 		shadowRoots?: ShadowRoot[];
 	}): string {
-		const xmlSerializer = new XMLSerializer();
-
-		if (options) {
-			if (options.serializableShadowRoots) {
-				xmlSerializer[PropertySymbol.options].serializableShadowRoots =
-					options.serializableShadowRoots;
-			}
-			if (options.shadowRoots) {
-				xmlSerializer[PropertySymbol.options].shadowRoots = options.shadowRoots;
-			}
-		}
+		const serializer = new HTMLSerializer({
+			serializableShadowRoots: !!options?.serializableShadowRoots,
+			shadowRoots: options?.shadowRoots ?? null
+		});
 
-		let xml = '';
+		let html = '';
 
 		for (const node of this[PropertySymbol.nodeArray]) {
-			xml += xmlSerializer.serializeToString(node);
+			html += serializer.serializeToString(node);
 		}
 
-		return xml;
+		return html;
 	}
 
 	/**
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 1f0b73a69..29f7efcc2 100644
--- a/packages/happy-dom/src/nodes/html-template-element/HTMLTemplateElement.ts
+++ b/packages/happy-dom/src/nodes/html-template-element/HTMLTemplateElement.ts
@@ -2,10 +2,9 @@ import HTMLElement from '../html-element/HTMLElement.js';
 import * as PropertySymbol from '../../PropertySymbol.js';
 import DocumentFragment from '../document-fragment/DocumentFragment.js';
 import Node from '../node/Node.js';
-import XMLSerializer from '../../xml-serializer/XMLSerializer.js';
 import XMLParser from '../../xml-parser/XMLParser.js';
 import ShadowRoot from '../shadow-root/ShadowRoot.js';
-import XMLParserModeEnum from '../../xml-parser/XMLParserModeEnum.js';
+import HTMLSerializer from '../../html-serializer/HTMLSerializer.js';
 
 /**
  * HTML Template Element.
@@ -70,18 +69,18 @@ export default class HTMLTemplateElement extends HTMLElement {
 	 * @override
 	 */
 	public override getInnerHTML(_options?: { includeShadowRoots?: boolean }): string {
-		const xmlSerializer = new XMLSerializer();
+		const serializer = new HTMLSerializer();
 
 		// Options should be ignored as shadow roots should not be serialized for HTMLTemplateElement.
 
 		const content = <DocumentFragment>this[PropertySymbol.content];
-		let xml = '';
+		let html = '';
 
 		for (const node of content[PropertySymbol.nodeArray]) {
-			xml += xmlSerializer.serializeToString(node);
+			html += serializer.serializeToString(node);
 		}
 
-		return xml;
+		return html;
 	}
 
 	/**
@@ -91,18 +90,18 @@ export default class HTMLTemplateElement extends HTMLElement {
 		serializableShadowRoots?: boolean;
 		shadowRoots?: ShadowRoot[];
 	}): string {
-		const xmlSerializer = new XMLSerializer();
+		const serializer = new HTMLSerializer();
 
 		// Options should be ignored as shadow roots should not be serialized for HTMLTemplateElement.
 
 		const content = <DocumentFragment>this[PropertySymbol.content];
-		let xml = '';
+		let html = '';
 
 		for (const node of content[PropertySymbol.nodeArray]) {
-			xml += xmlSerializer.serializeToString(node);
+			html += serializer.serializeToString(node);
 		}
 
-		return xml;
+		return html;
 	}
 
 	/**
diff --git a/packages/happy-dom/src/nodes/shadow-root/ShadowRoot.ts b/packages/happy-dom/src/nodes/shadow-root/ShadowRoot.ts
index b7b0ac476..35d36f942 100644
--- a/packages/happy-dom/src/nodes/shadow-root/ShadowRoot.ts
+++ b/packages/happy-dom/src/nodes/shadow-root/ShadowRoot.ts
@@ -1,14 +1,13 @@
 import DocumentFragment from '../document-fragment/DocumentFragment.js';
 import * as PropertySymbol from '../../PropertySymbol.js';
 import XMLParser from '../../xml-parser/XMLParser.js';
-import XMLSerializer from '../../xml-serializer/XMLSerializer.js';
 import Element from '../element/Element.js';
 import CSSStyleSheet from '../../css/CSSStyleSheet.js';
 import HTMLElement from '../../nodes/html-element/HTMLElement.js';
 import Event from '../../event/Event.js';
 import SVGElement from '../svg-element/SVGElement.js';
 import Document from '../document/Document.js';
-import XMLParserModeEnum from '../../xml-parser/XMLParserModeEnum.js';
+import HTMLSerializer from '../../html-serializer/HTMLSerializer.js';
 
 /**
  * ShadowRoot.
@@ -120,12 +119,12 @@ export default class ShadowRoot extends DocumentFragment {
 	 * @returns HTML.
 	 */
 	public get innerHTML(): string {
-		const xmlSerializer = new XMLSerializer();
-		let xml = '';
+		const serializer = new HTMLSerializer();
+		let html = '';
 		for (const node of this[PropertySymbol.nodeArray]) {
-			xml += xmlSerializer.serializeToString(node);
+			html += serializer.serializeToString(node);
 		}
-		return xml;
+		return html;
 	}
 
 	/**
diff --git a/packages/happy-dom/src/xml-serializer/XMLSerializer.ts b/packages/happy-dom/src/xml-serializer/XMLSerializer.ts
index 884b31ddc..b5ce89a33 100644
--- a/packages/happy-dom/src/xml-serializer/XMLSerializer.ts
+++ b/packages/happy-dom/src/xml-serializer/XMLSerializer.ts
@@ -4,12 +4,13 @@ import Node from '../nodes/node/Node.js';
 import DocumentType from '../nodes/document-type/DocumentType.js';
 import HTMLTemplateElement from '../nodes/html-template-element/HTMLTemplateElement.js';
 import NodeTypeEnum from '../nodes/node/NodeTypeEnum.js';
-import ProcessingInstruction from '../nodes/processing-instruction/ProcessingInstruction.js';
 import * as Entities from 'entities';
 import DocumentFragment from '../nodes/document-fragment/DocumentFragment.js';
-import ShadowRoot from '../nodes/shadow-root/ShadowRoot.js';
 import HTMLElementConfig from '../config/HTMLElementConfig.js';
 import HTMLElementConfigContentModelEnum from '../config/HTMLElementConfigContentModelEnum.js';
+import Document from '../nodes/document/Document.js';
+import ProcessingInstruction from '../nodes/processing-instruction/ProcessingInstruction.js';
+import XMLDocument from '../nodes/xml-document/XMLDocument.js';
 
 /**
  * Serializes a node into XML.
@@ -17,15 +18,7 @@ import HTMLElementConfigContentModelEnum from '../config/HTMLElementConfigConten
  * @see https://developer.mozilla.org/en-US/docs/Web/API/XMLSerializer
  */
 export default class XMLSerializer {
-	public [PropertySymbol.options]: {
-		serializableShadowRoots: boolean;
-		shadowRoots: ShadowRoot[] | null;
-		allShadowRoots: boolean;
-	} = {
-		serializableShadowRoots: false,
-		shadowRoots: null,
-		allShadowRoots: false
-	};
+	public [PropertySymbol.rootNode]: Node | null = null;
 
 	/**
 	 * Renders an element as HTML.
@@ -34,7 +27,9 @@ export default class XMLSerializer {
 	 * @returns Result.
 	 */
 	public serializeToString(root: Node): string {
-		const options = this[PropertySymbol.options];
+		if (!this[PropertySymbol.rootNode]) {
+			this[PropertySymbol.rootNode] = root;
+		}
 
 		switch (root[PropertySymbol.nodeType]) {
 			case NodeTypeEnum.elementNode:
@@ -48,24 +43,6 @@ export default class XMLSerializer {
 
 				let innerHTML = '';
 
-				// TODO: Should we include closed shadow roots? We are currently only including open shadow roots.
-				if (
-					element.shadowRoot &&
-					(options.allShadowRoots ||
-						(options.serializableShadowRoots && element.shadowRoot[PropertySymbol.serializable]) ||
-						options.shadowRoots?.includes(element.shadowRoot))
-				) {
-					innerHTML += `<template shadowrootmode="${element.shadowRoot[PropertySymbol.mode]}"${
-						element.shadowRoot[PropertySymbol.serializable] ? ' shadowrootserializable=""' : ''
-					}>`;
-
-					for (const node of (<ShadowRoot>element.shadowRoot)[PropertySymbol.nodeArray]) {
-						innerHTML += this.serializeToString(node);
-					}
-
-					innerHTML += '</template>';
-				}
-
 				const childNodes =
 					localName === 'template'
 						? (<DocumentFragment>(<HTMLTemplateElement>root).content)[PropertySymbol.nodeArray]
@@ -75,18 +52,14 @@ export default class XMLSerializer {
 					innerHTML += this.serializeToString(node);
 				}
 
-				// if (
-				// 	!innerHTML &&
-				// 	(root[PropertySymbol.namespaceURI] === NamespaceURI.xmlns ||
-				// 		root[PropertySymbol.namespaceURI] === NamespaceURI.svg)
-				// ) {
-				// 	return `<${localName}${this.getAttributes(element)}/>`;
-				// }
+				if (!innerHTML) {
+					return `<${localName}${this.getAttributes(element)}/>`;
+				}
 
 				return `<${localName}${this.getAttributes(element)}>${innerHTML}</${localName}>`;
 			case Node.DOCUMENT_FRAGMENT_NODE:
 			case Node.DOCUMENT_NODE:
-				let html = '';
+				let html = root instanceof XMLDocument ? '<?xml version="1.0" encoding="UTF-8"?>' : '';
 				for (const node of (<Node>root)[PropertySymbol.nodeArray]) {
 					html += this.serializeToString(node);
 				}
@@ -94,8 +67,7 @@ export default class XMLSerializer {
 			case NodeTypeEnum.commentNode:
 				return `<!--${root.textContent}-->`;
 			case NodeTypeEnum.processingInstructionNode:
-				// TODO: Add support for processing instructions.
-				return `<!--?${(<ProcessingInstruction>root).target} ${root.textContent}?-->`;
+				return `<?${(<ProcessingInstruction>root).target} ${root.textContent}?>`;
 			case NodeTypeEnum.textNode:
 				const parentElement = root.parentElement;
 				if (parentElement) {
@@ -125,38 +97,48 @@ export default class XMLSerializer {
 	private getAttributes(element: Element): string {
 		let attributeString = '';
 
+		const namedItems = (<Element>element)[PropertySymbol.attributes][PropertySymbol.namedItems];
+		const parentNamespaceURI = this.getNamespaceURI(this[PropertySymbol.parentNode]);
+		const namespaceURI = this.getNamespaceURI(element);
+
+		// We should add the namespace as an "xmlns" attribute if the element is the root element or if the namespace is different from the parent namespace.
 		if (
-			!(<Element>element)[PropertySymbol.attributes].getNamedItem('is') &&
-			(<Element>element)[PropertySymbol.isValue]
+			this[PropertySymbol.rootNode] === element ||
+			(parentNamespaceURI && namespaceURI !== parentNamespaceURI)
 		) {
-			attributeString += ' is="' + (<Element>element)[PropertySymbol.isValue] + '"';
+			if (namespaceURI) {
+				attributeString += ' xmlns="' + namespaceURI + '"';
+			}
 		}
 
-		const namedItems = (<Element>element)[PropertySymbol.attributes][PropertySymbol.namedItems];
-
-		// if (
-		// 	element[PropertySymbol.namespaceURI] === NamespaceURI.svg ||
-		// 	element[PropertySymbol.namespaceURI] === NamespaceURI.xmlns
-		// ) {
-		// 	const xmlns = namedItems.get('xmlns');
-
-		// 	// The "xmlns" attribute should always be the first attribute if it exists.
-		// 	if (xmlns && xmlns[PropertySymbol.value]) {
-		// 		attributeString += ' xmlns="' + xmlns[PropertySymbol.value] + '"';
-		// 	}
-		// }
+		if (!namedItems.has('is') && element[PropertySymbol.isValue]) {
+			attributeString += ' is="' + element[PropertySymbol.isValue] + '"';
+		}
 
 		for (const attribute of namedItems.values()) {
-			// if (
-			// 	attribute[PropertySymbol.name] !== 'xmlns' ||
-			// 	(element[PropertySymbol.namespaceURI] !== NamespaceURI.svg &&
-			// 		element[PropertySymbol.namespaceURI] !== NamespaceURI.xmlns)
-			// ) {
-			const escapedValue = Entities.escapeAttribute(attribute[PropertySymbol.value]);
-			attributeString += ' ' + attribute[PropertySymbol.name] + '="' + escapedValue + '"';
-			// }
+			// TODO: Attributes with the name "xmlns" are not serialized as they collide with the namespace attribute. Is this correct?
+			if (attribute[PropertySymbol.name] !== 'xmlns') {
+				const escapedValue = Entities.escapeAttribute(attribute[PropertySymbol.value]);
+				attributeString += ' ' + attribute[PropertySymbol.name] + '="' + escapedValue + '"';
+			}
 		}
 
 		return attributeString;
 	}
+
+	/**
+	 * Returns the namespace URI of a node.
+	 *
+	 * @param node Node.
+	 */
+	private getNamespaceURI(node: Node): string {
+		if (!node) {
+			return null;
+		}
+		if (node instanceof Document) {
+			return node.documentElement[PropertySymbol.namespaceURI];
+		}
+
+		return node[PropertySymbol.namespaceURI];
+	}
 }
diff --git a/packages/happy-dom/test/dom-parser/DOMParser.test.ts b/packages/happy-dom/test/dom-parser/DOMParser.test.ts
index 28f9db382..276d56871 100644
--- a/packages/happy-dom/test/dom-parser/DOMParser.test.ts
+++ b/packages/happy-dom/test/dom-parser/DOMParser.test.ts
@@ -1,9 +1,8 @@
 import Window from '../../src/window/Window.js';
-import XMLSerializer from '../../src/xml-serializer/XMLSerializer.js';
+import HTMLSerializer from '../../src/html-serializer/HTMLSerializer.js';
 import DOMParser from '../../src/dom-parser/DOMParser.js';
 import DOMParserHTML from './data/DOMParserHTML.js';
 import { beforeEach, describe, it, expect } from 'vitest';
-import DOMParserSVG from './data/DOMParserSVG';
 
 describe('DOMParser', () => {
 	let domParser: DOMParser;
@@ -24,14 +23,14 @@ describe('DOMParser', () => {
 	describe('parseFromString()', () => {
 		it('Parses HTML of a page and returns a new document.', () => {
 			const newDocument = domParser.parseFromString(DOMParserHTML, 'text/html');
-			expect(new XMLSerializer().serializeToString(newDocument).replace(/[\s]/gm, '')).toBe(
+			expect(new HTMLSerializer().serializeToString(newDocument).replace(/[\s]/gm, '')).toBe(
 				DOMParserHTML.replace(/[\s]/gm, '')
 			);
 		});
 
 		it('Parses HTML with just a string and returns a new document with <html>, <head> and <body> tags.', () => {
 			const newDocument = domParser.parseFromString('Test', 'text/html');
-			expect(new XMLSerializer().serializeToString(newDocument)).toBe(
+			expect(new HTMLSerializer().serializeToString(newDocument)).toBe(
 				'<!DOCTYPE html><html><head></head><body>Test</body></html>'
 			);
 		});
@@ -51,7 +50,7 @@ describe('DOMParser', () => {
 			`,
 				'text/html'
 			);
-			expect(new XMLSerializer().serializeToString(newDocument)).toBe(
+			expect(new HTMLSerializer().serializeToString(newDocument)).toBe(
 				`<!DOCTYPE html><html><head>
 						<title>Title</title>
 					</head>
@@ -76,7 +75,7 @@ describe('DOMParser', () => {
 				'text/html'
 			);
 			// Spurious comment `<!--[a-z]/,end:/-->` should be solved
-			expect(new XMLSerializer().serializeToString(newDocument)).toBe(
+			expect(new HTMLSerializer().serializeToString(newDocument)).toBe(
 				`<!DOCTYPE html><html><head></head><body>
 						<script>
 							var test = {className:"meta",begin:/<![a-z]/,end:/>/,contains:[t,i,l,c]};
@@ -103,7 +102,7 @@ describe('DOMParser', () => {
                 `,
 				'image/svg+xml'
 			);
-			expect(new XMLSerializer().serializeToString(newDocument))
+			expect(new HTMLSerializer().serializeToString(newDocument))
 				.toBe(`<svg xmlns="http://www.w3.org/2000/svg" width="14" height="14" viewBox="0 0 14 14" fill="none">
                         <path d="M14 6.53333V7.46667H7.46667V14H6.53333V7.46667H0V6.53333H6.53333V0H7.46667V6.53333H14Z" fill="#0078D4"/>
                     </svg>`);
@@ -138,7 +137,7 @@ describe('DOMParser', () => {
                 `,
 				'application/xml'
 			);
-			expect(new XMLSerializer().serializeToString(newDocument)).toBe(`<breakfast_menu>
+			expect(new HTMLSerializer().serializeToString(newDocument)).toBe(`<breakfast_menu>
                     <food>
                         <name>Belgian Waffles</name>
                         <price>$5.95</price>
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 4cc03a8e4..608c1be57 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
@@ -1,7 +1,7 @@
 import Window from '../../../src/window/Window.js';
 import Document from '../../../src/nodes/document/Document.js';
 import HTMLTemplateElement from '../../../src/nodes/html-template-element/HTMLTemplateElement.js';
-import XMLSerializer from '../../../src/xml-serializer/XMLSerializer.js';
+import HTMLSerializer from '../../../src/html-serializer/HTMLSerializer.js';
 import { beforeEach, afterEach, describe, it, expect, vi } from 'vitest';
 import CustomElement from '../../CustomElement.js';
 
@@ -40,7 +40,7 @@ describe('HTMLTemplateElement', () => {
 
 			expect(element.childNodes.length).toBe(0);
 			expect(element.innerHTML).toBe('<div>Test</div>');
-			expect(new XMLSerializer().serializeToString(element.content)).toBe('<div>Test</div>');
+			expect(new HTMLSerializer().serializeToString(element.content)).toBe('<div>Test</div>');
 
 			element.removeChild(div);
 
@@ -58,7 +58,7 @@ describe('HTMLTemplateElement', () => {
 
 			expect(element.childNodes.length).toBe(0);
 			expect(element.innerHTML).toBe('<div>Test</div>');
-			expect(new XMLSerializer().serializeToString(element.content)).toBe('<div>Test</div>');
+			expect(new HTMLSerializer().serializeToString(element.content)).toBe('<div>Test</div>');
 
 			element.innerHTML = '';
 
@@ -130,7 +130,7 @@ describe('HTMLTemplateElement', () => {
 
 			expect(element.childNodes.length).toBe(0);
 			expect(element.getInnerHTML()).toBe('<div>Test</div>');
-			expect(new XMLSerializer().serializeToString(element.content)).toBe('<div>Test</div>');
+			expect(new HTMLSerializer().serializeToString(element.content)).toBe('<div>Test</div>');
 
 			element.removeChild(div);
 
@@ -162,7 +162,7 @@ describe('HTMLTemplateElement', () => {
 
 			expect(element.childNodes.length).toBe(0);
 			expect(element.getHTML()).toBe('<div>Test</div>');
-			expect(new XMLSerializer().serializeToString(element.content)).toBe('<div>Test</div>');
+			expect(new HTMLSerializer().serializeToString(element.content)).toBe('<div>Test</div>');
 
 			element.removeChild(div);
 
diff --git a/packages/happy-dom/test/xml-parser/XMLParser.test.ts b/packages/happy-dom/test/xml-parser/XMLParser.test.ts
index 7b3695f2b..934c7e52b 100644
--- a/packages/happy-dom/test/xml-parser/XMLParser.test.ts
+++ b/packages/happy-dom/test/xml-parser/XMLParser.test.ts
@@ -12,6 +12,7 @@ import { beforeEach, describe, it, expect } from 'vitest';
 import CustomElement from '../CustomElement.js';
 import XMLParserModeEnum from '../../src/xml-parser/XMLParserModeEnum.js';
 import HTMLHtmlElement from '../../src/nodes/html-html-element/HTMLHtmlElement.js';
+import HTMLSerializer from '../../src/html-serializer/HTMLSerializer.js';
 
 describe('XMLParser', () => {
 	let window: Window;
@@ -159,7 +160,7 @@ describe('XMLParser', () => {
 	
 </body></html>`;
 			const root = new XMLParser(window, { mode: XMLParserModeEnum.htmlDocument }).parse(html);
-			expect(new XMLSerializer().serializeToString(root)).toBe(expected);
+			expect(new HTMLSerializer().serializeToString(root)).toBe(expected);
 		});
 
 		it('Parses a page with document type set to "HTML 4.01".', () => {
@@ -220,7 +221,7 @@ describe('XMLParser', () => {
 			expect(doctype.name).toBe('html');
 			expect(doctype.publicId).toBe('-//W3C//DTD HTML 4.01//EN');
 			expect(doctype.systemId).toBe('http://www.w3.org/TR/html4/strict.dtd');
-			expect(new XMLSerializer().serializeToString(root)).toBe(expected);
+			expect(new HTMLSerializer().serializeToString(root)).toBe(expected);
 		});
 
 		it('Handles unnestable elements correctly when there are siblings.', () => {
@@ -234,7 +235,7 @@ describe('XMLParser', () => {
                     <b>Test</b>
                 </article>`
 			);
-			expect(new XMLSerializer().serializeToString(root).replace(/\s/gm, '')).toBe(
+			expect(new HTMLSerializer().serializeToString(root).replace(/\s/gm, '')).toBe(
 				`
                 <article>
                     <span>
@@ -259,7 +260,7 @@ describe('XMLParser', () => {
                     <b>Test</b>
                 </article>`
 			);
-			expect(new XMLSerializer().serializeToString(root).replace(/\s/gm, '')).toBe(
+			expect(new HTMLSerializer().serializeToString(root).replace(/\s/gm, '')).toBe(
 				`
                 <article>
                     <span>
@@ -328,7 +329,7 @@ describe('XMLParser', () => {
 			expect(doctype.name).toBe('math');
 			expect(doctype.publicId).toBe('');
 			expect(doctype.systemId).toBe('http://www.w3.org/Math/DTD/mathml1/mathml.dtd');
-			expect(new XMLSerializer().serializeToString(root)).toBe(expected);
+			expect(new HTMLSerializer().serializeToString(root)).toBe(expected);
 		});
 
 		it('Handles unclosed tags of unnestable elements (e.g. <a>, <li>).', () => {
@@ -345,7 +346,7 @@ describe('XMLParser', () => {
 				`
 			);
 
-			expect(new XMLSerializer().serializeToString(root).replace(/\s/gm, '')).toBe(
+			expect(new HTMLSerializer().serializeToString(root).replace(/\s/gm, '')).toBe(
 				`
 				<div class="test" disabled="">
 					<ul>
@@ -375,7 +376,7 @@ describe('XMLParser', () => {
 			expect((<HTMLElement>root.children[0].children[1]).innerText).toBe('<b></b>');
 			expect((<HTMLElement>root.children[0].children[2]).innerText).toBe('<b></b>');
 
-			expect(new XMLSerializer().serializeToString(root)).toBe(
+			expect(new HTMLSerializer().serializeToString(root)).toBe(
 				`<div>
 					<script>if(1<Math['random']()){}else if(Math['random']()>1){console.log("1")}</script>
 					<script><b></b></script>
@@ -491,7 +492,7 @@ describe('XMLParser', () => {
 			expect(svg.attributes['xmlns'].ownerElement === svg).toBe(true);
 			expect(svg.attributes['xmlns'].ownerDocument === document).toBe(true);
 
-			expect(new XMLSerializer().serializeToString(root)).toBe(
+			expect(new HTMLSerializer().serializeToString(root)).toBe(
 				`
 				<div>
 					<svg viewBox="0 0 300 100" stroke="red" fill="grey" xmlns="${NamespaceURI.html}">
@@ -533,7 +534,7 @@ describe('XMLParser', () => {
 			`
 			);
 
-			expect(new XMLSerializer().serializeToString(root)).toBe(
+			expect(new HTMLSerializer().serializeToString(root)).toBe(
 				`
 				<div>
 					<svg viewBox="0 0 300 100" stroke="red" fill="grey" xmlns="http://www.w3.org/1999/xhtml">
@@ -569,16 +570,16 @@ describe('XMLParser', () => {
 
 		it('Handles different value types.', () => {
 			const root1 = new XMLParser(window).parse(<string>(<unknown>null));
-			expect(new XMLSerializer().serializeToString(root1)).toBe('');
+			expect(new HTMLSerializer().serializeToString(root1)).toBe('');
 
 			const root2 = new XMLParser(window).parse(<string>(<unknown>undefined));
-			expect(new XMLSerializer().serializeToString(root2)).toBe('');
+			expect(new HTMLSerializer().serializeToString(root2)).toBe('');
 
 			const root3 = new XMLParser(window).parse(<string>(<unknown>1000));
-			expect(new XMLSerializer().serializeToString(root3)).toBe('1000');
+			expect(new HTMLSerializer().serializeToString(root3)).toBe('1000');
 
 			const root4 = new XMLParser(window).parse(<string>(<unknown>false));
-			expect(new XMLSerializer().serializeToString(root4)).toBe('false');
+			expect(new HTMLSerializer().serializeToString(root4)).toBe('false');
 		});
 
 		it('Parses conditional comments', () => {
@@ -615,7 +616,7 @@ describe('XMLParser', () => {
 
 			for (const html of testHTML) {
 				const root = new XMLParser(window, { mode: XMLParserModeEnum.htmlFragment }).parse(html);
-				expect(new XMLSerializer().serializeToString(root)).toBe(html);
+				expect(new HTMLSerializer().serializeToString(root)).toBe(html);
 			}
 		});
 
@@ -664,7 +665,7 @@ describe('XMLParser', () => {
 				'</html>';
 
 			const root = new XMLParser(window, { mode: XMLParserModeEnum.htmlDocument }).parse(html);
-			expect(new XMLSerializer().serializeToString(root)).toBe(expected);
+			expect(new HTMLSerializer().serializeToString(root)).toBe(expected);
 		});
 
 		it('Parses comments with dash in them.', () => {
@@ -712,7 +713,7 @@ describe('XMLParser', () => {
                 `
 			);
 
-			expect(new XMLSerializer().serializeToString(root).replace(/\s/gm, '')).toBe(
+			expect(new HTMLSerializer().serializeToString(root).replace(/\s/gm, '')).toBe(
 				`
                 <span key1="value1">
                     <span key1="value1" key2="">
@@ -725,7 +726,7 @@ describe('XMLParser', () => {
 		it('Can read text with ">" in it.', () => {
 			const root = new XMLParser(window).parse(`<span>1 > 0</span>`);
 
-			expect(new XMLSerializer().serializeToString(root)).toBe(`<span>1 &gt; 0</span>`);
+			expect(new HTMLSerializer().serializeToString(root)).toBe(`<span>1 &gt; 0</span>`);
 		});
 
 		it('Parses malformed attributes.', () => {
@@ -820,7 +821,7 @@ describe('XMLParser', () => {
 				`<div .theme$lit$={{lit-12345}} key1="value1">Test</div>`
 			);
 
-			expect(new XMLSerializer().serializeToString(root)).toBe(
+			expect(new HTMLSerializer().serializeToString(root)).toBe(
 				'<div .theme$lit$="{{lit-12345}}" key1="value1">Test</div>'
 			);
 		});
@@ -828,7 +829,7 @@ describe('XMLParser', () => {
 		it('Parses attributes with URL without apostrophs.', () => {
 			const root = new XMLParser(window).parse(`<a href=http://www.github.com/path>Click me</a>`);
 
-			expect(new XMLSerializer().serializeToString(root)).toBe(
+			expect(new HTMLSerializer().serializeToString(root)).toBe(
 				'<a href="http://www.github.com/path">Click me</a>'
 			);
 		});
@@ -836,7 +837,7 @@ describe('XMLParser', () => {
 		it('Parses attributes with single apostrophs.', () => {
 			const root = new XMLParser(window).parse(`<div key1='value1' key2='value2'>Test</div>`);
 
-			expect(new XMLSerializer().serializeToString(root)).toBe(
+			expect(new HTMLSerializer().serializeToString(root)).toBe(
 				`<div key1="value1" key2="value2">Test</div>`
 			);
 		});
@@ -851,7 +852,7 @@ describe('XMLParser', () => {
                 `
 			);
 
-			expect(new XMLSerializer().serializeToString(root).replace(/\s/gm, '')).toBe(
+			expect(new HTMLSerializer().serializeToString(root).replace(/\s/gm, '')).toBe(
 				'<divkey1="value1">Test</div>'
 			);
 			expect(root.children[0].textContent.replace(/\s/gm, '')).toBe('Test');
@@ -871,7 +872,7 @@ describe('XMLParser', () => {
                 </div>`
 			);
 
-			expect(new XMLSerializer().serializeToString(root).replace(/\s/gm, '')).toBe(
+			expect(new HTMLSerializer().serializeToString(root).replace(/\s/gm, '')).toBe(
 				`<div>
                     <ul>
                         <li>
@@ -886,7 +887,7 @@ describe('XMLParser', () => {
 
 			const root2 = new XMLParser(window).parse(`<li><li><span>Test</span></li></li>`);
 
-			expect(new XMLSerializer().serializeToString(root2).replace(/\s/gm, '')).toBe(
+			expect(new HTMLSerializer().serializeToString(root2).replace(/\s/gm, '')).toBe(
 				`<li></li><li><span>Test</span></li>`.replace(/\s/gm, '')
 			);
 		});
@@ -905,7 +906,7 @@ describe('XMLParser', () => {
                 </div>`
 			);
 
-			expect(new XMLSerializer().serializeToString(root)).toBe(
+			expect(new HTMLSerializer().serializeToString(root)).toBe(
 				`<div>
                     <ul>
                         <li>
@@ -933,7 +934,7 @@ describe('XMLParser', () => {
 			    </div>`
 			);
 
-			expect(new XMLSerializer().serializeToString(root).replace(/\s/gm, '')).toBe(
+			expect(new HTMLSerializer().serializeToString(root).replace(/\s/gm, '')).toBe(
 				`<div class="container">
                     <div class="sliderContainer">
                         <input class="slider" type="range" step="1" min="0" max="100">
@@ -961,7 +962,7 @@ describe('XMLParser', () => {
                 `
 			);
 
-			expect(new XMLSerializer().serializeToString(root).replace(/\s/gm, '')).toBe(
+			expect(new HTMLSerializer().serializeToString(root).replace(/\s/gm, '')).toBe(
 				`
                 <div>
                     <a-Öa key1="value1" key2="value2">
@@ -1002,7 +1003,7 @@ describe('XMLParser', () => {
 				'<!DOCTYPE html><html><head></head><body><div>Test</div></body></html>'
 			);
 
-			expect(new XMLSerializer().serializeToString(root)).toBe('<div>Test</div>');
+			expect(new HTMLSerializer().serializeToString(root)).toBe('<div>Test</div>');
 		});
 
 		it('Handles multiple <!DOCTYPE>.', () => {
@@ -1038,7 +1039,7 @@ describe('XMLParser', () => {
                 </html>`
 			);
 
-			expect(new XMLSerializer().serializeToString(root)).toBe(
+			expect(new HTMLSerializer().serializeToString(root)).toBe(
 				`<!DOCTYPE html><html style="color: red"><head></head>
                     <body><div>Test</div>
                 
@@ -1059,7 +1060,7 @@ describe('XMLParser', () => {
                 </head>`
 			);
 
-			expect(new XMLSerializer().serializeToString(root))
+			expect(new HTMLSerializer().serializeToString(root))
 				.toBe(`<!DOCTYPE html><html><head style="color: red">
                     <title>Title 1</title>
                 <title>Title 2</title></head>
@@ -1079,7 +1080,7 @@ describe('XMLParser', () => {
                 </head>`
 			);
 
-			expect(new XMLSerializer().serializeToString(root)).toBe(`<!DOCTYPE html><html><head>
+			expect(new HTMLSerializer().serializeToString(root)).toBe(`<!DOCTYPE html><html><head>
                     <title>Title 1</title>
                 </head>
                 <body>
@@ -1098,7 +1099,7 @@ describe('XMLParser', () => {
                 </body>`
 			);
 
-			expect(new XMLSerializer().serializeToString(root1))
+			expect(new HTMLSerializer().serializeToString(root1))
 				.toBe(`<!DOCTYPE html><html><head></head><body style="color: red">
                     <div>Test 1</div>
                 
@@ -1120,7 +1121,7 @@ describe('XMLParser', () => {
                 </html>`
 			);
 
-			expect(new XMLSerializer().serializeToString(root2)).toBe(`<!DOCTYPE html><html><head>
+			expect(new HTMLSerializer().serializeToString(root2)).toBe(`<!DOCTYPE html><html><head>
                         <title>Title</title>
                     </head>
                     <body style="color: red">
@@ -1132,7 +1133,7 @@ describe('XMLParser', () => {
                 </body></html>`);
 		});
 
-		it('Handles processing instructions as comments.', () => {
+		it('Handles processing instructions as comments when parser mode is HTML.', () => {
 			const root = new XMLParser(window).parse(
 				`<?xml version="1.0" encoding="UTF-8"?>
                 <div>
@@ -1140,7 +1141,7 @@ describe('XMLParser', () => {
                 </div>`
 			);
 
-			expect(new XMLSerializer().serializeToString(root)).toBe(
+			expect(new HTMLSerializer().serializeToString(root)).toBe(
 				`<!--?xml version="1.0" encoding="UTF-8"?-->
                 <div>
                     <div>Test</div>
@@ -1148,6 +1149,21 @@ describe('XMLParser', () => {
 			);
 		});
 
+		it('Ignores processing instructions as comments when parser mode is XML.', () => {
+			const root = new XMLParser(window, { mode: XMLParserModeEnum.xmlDocument }).parse(
+				`<?xml version="1.0" encoding="UTF-8"?>
+                <div>
+                    <div>Test</div>
+                </div>`
+			);
+
+			expect(new XMLSerializer().serializeToString(root)).toBe(
+				`<div>
+                    <div>Test</div>
+                </div>`
+			);
+		});
+
 		it('Handles namespaced XML', () => {
 			const root = new XMLParser(window, { mode: XMLParserModeEnum.xmlDocument }).parse(
 				`<?xml version="1.0" encoding="UTF-8"?>
@@ -1163,7 +1179,7 @@ describe('XMLParser', () => {
 			);
 
 			expect(new XMLSerializer().serializeToString(root)).toBe(
-				`<personxml:person xmlns:personxml="http://www.your.example.com/xml/person" xmlns:cityxml="http://www.my.example.com/xml/cities">
+				`<?xml version="1.0" encoding="UTF-8"?><personxml:person xmlns:personxml="http://www.your.example.com/xml/person" xmlns:cityxml="http://www.my.example.com/xml/cities">
                 <personxml:name>Rob</personxml:name>
                 <personxml:age>37</personxml:age>
                 <cityxml:homecity>