diff --git a/packages/happy-dom/src/PropertySymbol.ts b/packages/happy-dom/src/PropertySymbol.ts
index be9bee7b5..c19a2afef 100644
--- a/packages/happy-dom/src/PropertySymbol.ts
+++ b/packages/happy-dom/src/PropertySymbol.ts
@@ -82,7 +82,6 @@ export const scrollWidth = Symbol('scrollWidth');
 export const scrollTop = Symbol('scrollTop');
 export const scrollLeft = Symbol('scrollLeft');
 export const attributes = Symbol('attributes');
-export const attributesProxy = Symbol('attributesProxy');
 export const namespaceURI = Symbol('namespaceURI');
 export const accessKey = Symbol('accessKey');
 export const accessKeyLabel = Symbol('accessKeyLabel');
@@ -376,6 +375,7 @@ export const rotate = Symbol('rotate');
 export const bindMethods = Symbol('bindMethods');
 export const xmlProcessingInstruction = Symbol('xmlProcessingInstruction');
 export const root = Symbol('root');
-export const filterNode = Symbol('filterNode');
 export const customElementReactionStack = Symbol('customElementReactionStack');
+export const isRemoved = Symbol('isRemoved');
+export const currentNode = Symbol('currentNode');
 export const dispatching = Symbol('dispatching');
diff --git a/packages/happy-dom/src/config/HTMLElementConfig.ts b/packages/happy-dom/src/config/HTMLElementConfig.ts
index 76fb7d69a..51536ee34 100644
--- a/packages/happy-dom/src/config/HTMLElementConfig.ts
+++ b/packages/happy-dom/src/config/HTMLElementConfig.ts
@@ -13,6 +13,7 @@ export default <
 			permittedParents?: string[];
 			addPermittedParent?: string;
 			moveForbiddenDescendant?: { exclude: string[] };
+			escapesSVGNamespace?: boolean;
 		};
 	}
 >{
@@ -46,7 +47,8 @@ export default <
 	},
 	b: {
 		className: 'HTMLElement',
-		contentModel: HTMLElementConfigContentModelEnum.anyDescendants
+		contentModel: HTMLElementConfigContentModelEnum.anyDescendants,
+		escapesSVGNamespace: true
 	},
 	base: {
 		className: 'HTMLBaseElement',
@@ -62,7 +64,8 @@ export default <
 	},
 	body: {
 		className: 'HTMLBodyElement',
-		contentModel: HTMLElementConfigContentModelEnum.anyDescendants
+		contentModel: HTMLElementConfigContentModelEnum.anyDescendants,
+		escapesSVGNamespace: true
 	},
 	template: {
 		className: 'HTMLTemplateElement',
@@ -86,7 +89,8 @@ export default <
 	},
 	img: {
 		className: 'HTMLImageElement',
-		contentModel: HTMLElementConfigContentModelEnum.noDescendants
+		contentModel: HTMLElementConfigContentModelEnum.noDescendants,
+		escapesSVGNamespace: true
 	},
 	link: {
 		className: 'HTMLLinkElement',
@@ -106,15 +110,18 @@ export default <
 	},
 	meta: {
 		className: 'HTMLMetaElement',
-		contentModel: HTMLElementConfigContentModelEnum.noDescendants
+		contentModel: HTMLElementConfigContentModelEnum.noDescendants,
+		escapesSVGNamespace: true
 	},
 	blockquote: {
 		className: 'HTMLQuoteElement',
-		contentModel: HTMLElementConfigContentModelEnum.anyDescendants
+		contentModel: HTMLElementConfigContentModelEnum.anyDescendants,
+		escapesSVGNamespace: true
 	},
 	br: {
 		className: 'HTMLBRElement',
-		contentModel: HTMLElementConfigContentModelEnum.noDescendants
+		contentModel: HTMLElementConfigContentModelEnum.noDescendants,
+		escapesSVGNamespace: true
 	},
 	button: {
 		className: 'HTMLButtonElement',
@@ -134,7 +141,8 @@ export default <
 	},
 	code: {
 		className: 'HTMLElement',
-		contentModel: HTMLElementConfigContentModelEnum.anyDescendants
+		contentModel: HTMLElementConfigContentModelEnum.anyDescendants,
+		escapesSVGNamespace: true
 	},
 	col: {
 		className: 'HTMLTableColElement',
@@ -157,7 +165,8 @@ export default <
 	dd: {
 		className: 'HTMLElement',
 		contentModel: HTMLElementConfigContentModelEnum.noForbiddenFirstLevelDescendants,
-		forbiddenDescendants: ['dt', 'dd']
+		forbiddenDescendants: ['dt', 'dd'],
+		escapesSVGNamespace: true
 	},
 	del: {
 		className: 'HTMLModElement',
@@ -177,24 +186,29 @@ export default <
 	},
 	div: {
 		className: 'HTMLDivElement',
-		contentModel: HTMLElementConfigContentModelEnum.anyDescendants
+		contentModel: HTMLElementConfigContentModelEnum.anyDescendants,
+		escapesSVGNamespace: true
 	},
 	dl: {
 		className: 'HTMLDListElement',
-		contentModel: HTMLElementConfigContentModelEnum.anyDescendants
+		contentModel: HTMLElementConfigContentModelEnum.anyDescendants,
+		escapesSVGNamespace: true
 	},
 	dt: {
 		className: 'HTMLElement',
 		contentModel: HTMLElementConfigContentModelEnum.noForbiddenFirstLevelDescendants,
-		forbiddenDescendants: ['dt', 'dd']
+		forbiddenDescendants: ['dt', 'dd'],
+		escapesSVGNamespace: true
 	},
 	em: {
 		className: 'HTMLElement',
-		contentModel: HTMLElementConfigContentModelEnum.anyDescendants
+		contentModel: HTMLElementConfigContentModelEnum.anyDescendants,
+		escapesSVGNamespace: true
 	},
 	embed: {
 		className: 'HTMLEmbedElement',
-		contentModel: HTMLElementConfigContentModelEnum.noDescendants
+		contentModel: HTMLElementConfigContentModelEnum.noDescendants,
+		escapesSVGNamespace: true
 	},
 	fieldset: {
 		className: 'HTMLFieldSetElement',
@@ -214,31 +228,38 @@ export default <
 	},
 	h1: {
 		className: 'HTMLHeadingElement',
-		contentModel: HTMLElementConfigContentModelEnum.noFirstLevelSelfDescendants
+		contentModel: HTMLElementConfigContentModelEnum.noFirstLevelSelfDescendants,
+		escapesSVGNamespace: true
 	},
 	h2: {
 		className: 'HTMLHeadingElement',
-		contentModel: HTMLElementConfigContentModelEnum.noFirstLevelSelfDescendants
+		contentModel: HTMLElementConfigContentModelEnum.noFirstLevelSelfDescendants,
+		escapesSVGNamespace: true
 	},
 	h3: {
 		className: 'HTMLHeadingElement',
-		contentModel: HTMLElementConfigContentModelEnum.noFirstLevelSelfDescendants
+		contentModel: HTMLElementConfigContentModelEnum.noFirstLevelSelfDescendants,
+		escapesSVGNamespace: true
 	},
 	h4: {
 		className: 'HTMLHeadingElement',
-		contentModel: HTMLElementConfigContentModelEnum.noFirstLevelSelfDescendants
+		contentModel: HTMLElementConfigContentModelEnum.noFirstLevelSelfDescendants,
+		escapesSVGNamespace: true
 	},
 	h5: {
 		className: 'HTMLHeadingElement',
-		contentModel: HTMLElementConfigContentModelEnum.noFirstLevelSelfDescendants
+		contentModel: HTMLElementConfigContentModelEnum.noFirstLevelSelfDescendants,
+		escapesSVGNamespace: true
 	},
 	h6: {
 		className: 'HTMLHeadingElement',
-		contentModel: HTMLElementConfigContentModelEnum.noFirstLevelSelfDescendants
+		contentModel: HTMLElementConfigContentModelEnum.noFirstLevelSelfDescendants,
+		escapesSVGNamespace: true
 	},
 	head: {
 		className: 'HTMLHeadElement',
-		contentModel: HTMLElementConfigContentModelEnum.anyDescendants
+		contentModel: HTMLElementConfigContentModelEnum.anyDescendants,
+		escapesSVGNamespace: true
 	},
 	header: {
 		className: 'HTMLElement',
@@ -250,7 +271,8 @@ export default <
 	},
 	hr: {
 		className: 'HTMLHRElement',
-		contentModel: HTMLElementConfigContentModelEnum.noDescendants
+		contentModel: HTMLElementConfigContentModelEnum.noDescendants,
+		escapesSVGNamespace: true
 	},
 	html: {
 		className: 'HTMLHtmlElement',
@@ -258,7 +280,8 @@ export default <
 	},
 	i: {
 		className: 'HTMLElement',
-		contentModel: HTMLElementConfigContentModelEnum.anyDescendants
+		contentModel: HTMLElementConfigContentModelEnum.anyDescendants,
+		escapesSVGNamespace: true
 	},
 	iframe: {
 		className: 'HTMLIFrameElement',
@@ -278,7 +301,8 @@ export default <
 	},
 	li: {
 		className: 'HTMLLIElement',
-		contentModel: HTMLElementConfigContentModelEnum.noFirstLevelSelfDescendants
+		contentModel: HTMLElementConfigContentModelEnum.noFirstLevelSelfDescendants,
+		escapesSVGNamespace: true
 	},
 	main: {
 		className: 'HTMLElement',
@@ -294,7 +318,8 @@ export default <
 	},
 	menu: {
 		className: 'HTMLMenuElement',
-		contentModel: HTMLElementConfigContentModelEnum.anyDescendants
+		contentModel: HTMLElementConfigContentModelEnum.anyDescendants,
+		escapesSVGNamespace: true
 	},
 	meter: {
 		className: 'HTMLMeterElement',
@@ -314,7 +339,8 @@ export default <
 	},
 	ol: {
 		className: 'HTMLOListElement',
-		contentModel: HTMLElementConfigContentModelEnum.anyDescendants
+		contentModel: HTMLElementConfigContentModelEnum.anyDescendants,
+		escapesSVGNamespace: true
 	},
 	optgroup: {
 		className: 'HTMLOptGroupElement',
@@ -331,7 +357,8 @@ export default <
 	},
 	p: {
 		className: 'HTMLParagraphElement',
-		contentModel: HTMLElementConfigContentModelEnum.anyDescendants
+		contentModel: HTMLElementConfigContentModelEnum.anyDescendants,
+		escapesSVGNamespace: true
 	},
 	param: {
 		className: 'HTMLParamElement',
@@ -343,7 +370,8 @@ export default <
 	},
 	pre: {
 		className: 'HTMLPreElement',
-		contentModel: HTMLElementConfigContentModelEnum.anyDescendants
+		contentModel: HTMLElementConfigContentModelEnum.anyDescendants,
+		escapesSVGNamespace: true
 	},
 	progress: {
 		className: 'HTMLProgressElement',
@@ -373,11 +401,13 @@ export default <
 	},
 	ruby: {
 		className: 'HTMLElement',
-		contentModel: HTMLElementConfigContentModelEnum.anyDescendants
+		contentModel: HTMLElementConfigContentModelEnum.anyDescendants,
+		escapesSVGNamespace: true
 	},
 	s: {
 		className: 'HTMLElement',
-		contentModel: HTMLElementConfigContentModelEnum.anyDescendants
+		contentModel: HTMLElementConfigContentModelEnum.anyDescendants,
+		escapesSVGNamespace: true
 	},
 	samp: {
 		className: 'HTMLElement',
@@ -393,7 +423,8 @@ export default <
 	},
 	small: {
 		className: 'HTMLElement',
-		contentModel: HTMLElementConfigContentModelEnum.anyDescendants
+		contentModel: HTMLElementConfigContentModelEnum.anyDescendants,
+		escapesSVGNamespace: true
 	},
 	source: {
 		className: 'HTMLSourceElement',
@@ -401,15 +432,18 @@ export default <
 	},
 	span: {
 		className: 'HTMLSpanElement',
-		contentModel: HTMLElementConfigContentModelEnum.anyDescendants
+		contentModel: HTMLElementConfigContentModelEnum.anyDescendants,
+		escapesSVGNamespace: true
 	},
 	strong: {
 		className: 'HTMLElement',
-		contentModel: HTMLElementConfigContentModelEnum.anyDescendants
+		contentModel: HTMLElementConfigContentModelEnum.anyDescendants,
+		escapesSVGNamespace: true
 	},
 	sub: {
 		className: 'HTMLElement',
-		contentModel: HTMLElementConfigContentModelEnum.anyDescendants
+		contentModel: HTMLElementConfigContentModelEnum.anyDescendants,
+		escapesSVGNamespace: true
 	},
 	summary: {
 		className: 'HTMLElement',
@@ -417,13 +451,15 @@ export default <
 	},
 	sup: {
 		className: 'HTMLElement',
-		contentModel: HTMLElementConfigContentModelEnum.anyDescendants
+		contentModel: HTMLElementConfigContentModelEnum.anyDescendants,
+		escapesSVGNamespace: true
 	},
 	table: {
 		className: 'HTMLTableElement',
 		contentModel: HTMLElementConfigContentModelEnum.permittedDescendants,
 		permittedDescendants: ['caption', 'colgroup', 'thead', 'tfoot', 'tbody'],
-		moveForbiddenDescendant: { exclude: [] }
+		moveForbiddenDescendant: { exclude: [] },
+		escapesSVGNamespace: true
 	},
 	tbody: {
 		className: 'HTMLTableSectionElement',
@@ -480,15 +516,18 @@ export default <
 	},
 	u: {
 		className: 'HTMLElement',
-		contentModel: HTMLElementConfigContentModelEnum.anyDescendants
+		contentModel: HTMLElementConfigContentModelEnum.anyDescendants,
+		escapesSVGNamespace: true
 	},
 	ul: {
 		className: 'HTMLUListElement',
-		contentModel: HTMLElementConfigContentModelEnum.anyDescendants
+		contentModel: HTMLElementConfigContentModelEnum.anyDescendants,
+		escapesSVGNamespace: true
 	},
 	var: {
 		className: 'HTMLElement',
-		contentModel: HTMLElementConfigContentModelEnum.anyDescendants
+		contentModel: HTMLElementConfigContentModelEnum.anyDescendants,
+		escapesSVGNamespace: true
 	},
 	video: {
 		className: 'HTMLVideoElement',
diff --git a/packages/happy-dom/src/config/MathMLElementConfig.ts b/packages/happy-dom/src/config/MathMLElementConfig.ts
new file mode 100644
index 000000000..7da8f59e3
--- /dev/null
+++ b/packages/happy-dom/src/config/MathMLElementConfig.ts
@@ -0,0 +1,44 @@
+/**
+ * @see https://w3c.github.io/mathml-core/
+ * @see https://developer.mozilla.org/en-US/docs/Web/MathML/Element
+ */
+export default <
+	{
+		[key: string]: boolean;
+	}
+>{
+	'annotation-xml': true,
+	annotation: true,
+	maction: true,
+	math: true,
+	menclose: true,
+	merror: true,
+	mfenced: true,
+	mfrac: true,
+	mglyph: true,
+	mi: true,
+	mlabeledtr: true,
+	mmultiscripts: true,
+	mn: true,
+	mo: true,
+	mover: true,
+	mpadded: true,
+	mphantom: true,
+	mprescripts: true,
+	mroot: true,
+	mrow: true,
+	ms: true,
+	mspace: true,
+	msqrt: true,
+	mstyle: true,
+	msub: true,
+	msubsup: true,
+	msup: true,
+	mtable: true,
+	mtd: true,
+	mtext: true,
+	mtr: true,
+	munder: true,
+	munderover: true,
+	semantics: true
+};
diff --git a/packages/happy-dom/src/html-parser/HTMLParser.ts b/packages/happy-dom/src/html-parser/HTMLParser.ts
index c5f2ab463..31b41455d 100755
--- a/packages/happy-dom/src/html-parser/HTMLParser.ts
+++ b/packages/happy-dom/src/html-parser/HTMLParser.ts
@@ -17,6 +17,7 @@ import HTMLBodyElement from '../nodes/html-body-element/HTMLBodyElement.js';
 import HTMLHtmlElement from '../nodes/html-html-element/HTMLHtmlElement.js';
 import XMLEncodeUtility from '../utilities/XMLEncodeUtility.js';
 import NodeTypeEnum from '../nodes/node/NodeTypeEnum.js';
+import MathMLElementConfig from '../config/MathMLElementConfig.js';
 
 /**
  * Markup RegExp.
@@ -434,12 +435,18 @@ export default class HTMLParser {
 
 		const tagName = this.nextElement[PropertySymbol.tagName];
 		const lowerTagName = tagName.toLowerCase();
-		const config = HTMLElementConfig[lowerTagName];
+		const config =
+			this.nextElement[PropertySymbol.namespaceURI] === NamespaceURI.html
+				? HTMLElementConfig[lowerTagName]
+				: null;
 		let previousCurrentNode: Node | null = null;
 
 		while (previousCurrentNode !== this.rootNode) {
 			const parentLowerTagName = this.currentNode[PropertySymbol.tagName]?.toLowerCase();
-			const parentConfig = HTMLElementConfig[parentLowerTagName];
+			const parentConfig =
+				this.currentNode[PropertySymbol.namespaceURI] === NamespaceURI.html
+					? HTMLElementConfig[parentLowerTagName]
+					: null;
 
 			if (previousCurrentNode === this.currentNode) {
 				throw new Error(
@@ -513,17 +520,7 @@ export default class HTMLParser {
 				config?.contentModel === HTMLElementConfigContentModelEnum.noSelfDescendants &&
 				this.tagNameStack.includes(tagName)
 			) {
-				while (this.currentNode !== this.rootNode) {
-					if ((<Element>this.currentNode)[PropertySymbol.tagName] === tagName) {
-						this.nodeStack.pop();
-						this.tagNameStack.pop();
-						this.currentNode = this.nodeStack[this.nodeStack.length - 1] || this.rootNode;
-						break;
-					}
-					this.nodeStack.pop();
-					this.tagNameStack.pop();
-					this.currentNode = this.nodeStack[this.nodeStack.length - 1] || this.rootNode;
-				}
+				this.parseEndTag(tagName);
 			} else if (
 				config?.permittedParents &&
 				!config.permittedParents.includes(parentLowerTagName)
@@ -746,7 +743,10 @@ export default class HTMLParser {
 	 * @param text Text.
 	 */
 	private parseRawTextElementContent(tagName: string, text: string): void {
-		const upperTagName = StringUtility.asciiUpperCase(tagName);
+		const upperTagName =
+			this.currentNode[PropertySymbol.namespaceURI] === NamespaceURI.html
+				? StringUtility.asciiUpperCase(tagName)
+				: tagName;
 
 		if (upperTagName !== this.currentNode[PropertySymbol.tagName]) {
 			return;
@@ -800,19 +800,43 @@ export default class HTMLParser {
 	private getStartTagElement(tagName: string): Element {
 		const lowerTagName = StringUtility.asciiLowerCase(tagName);
 
-		// NamespaceURI is inherited from the parent element.
+		// Namespace URI is inherited from the parent element.
 		const namespaceURI = this.currentNode[PropertySymbol.namespaceURI];
 
-		// NamespaceURI should be SVG when the tag name is "svg" (even in XML mode).
+		// Namespace URI should be MathML when the tag name is "math".
+		if (lowerTagName === 'math' || namespaceURI === NamespaceURI.mathML) {
+			// <mi> is a special case where children of it can escape the Math ML namespace if it isn't a known Math ML element.
+			if (this.currentNode[PropertySymbol.tagName] === 'mi' && !MathMLElementConfig[lowerTagName]) {
+				if (lowerTagName === 'svg') {
+					return this.rootDocument.createElementNS(NamespaceURI.svg, 'svg');
+				}
+				return this.rootDocument.createElementNS(NamespaceURI.html, lowerTagName);
+			}
+			return this.rootDocument.createElementNS(NamespaceURI.mathML, tagName);
+		}
+
+		// Namespace URI should be SVG when the tag name is "svg".
 		if (lowerTagName === 'svg') {
 			return this.rootDocument.createElementNS(NamespaceURI.svg, 'svg');
 		}
 
 		if (namespaceURI === NamespaceURI.svg) {
-			return this.rootDocument.createElementNS(
-				NamespaceURI.svg,
-				SVGElementConfig[lowerTagName]?.localName || tagName
-			);
+			// <foreignObject> is a special case where children of it escapes the SVG namespace.
+			// @see https://developer.mozilla.org/en-US/docs/Web/SVG/Element/foreignObject
+			if (this.currentNode[PropertySymbol.tagName] === 'foreignObject') {
+				return this.rootDocument.createElementNS(NamespaceURI.html, lowerTagName);
+			}
+			const config = SVGElementConfig[lowerTagName];
+			if (config) {
+				return this.rootDocument.createElementNS(NamespaceURI.svg, config.localName);
+			}
+			// Some HTML tags escapes the SVG namespace
+			// We should then end all SVG elements up to the first matching <svg> tag
+			if (HTMLElementConfig[lowerTagName]?.escapesSVGNamespace) {
+				this.parseEndTag('svg');
+			} else {
+				return this.rootDocument.createElementNS(NamespaceURI.svg, tagName);
+			}
 		}
 
 		// New element.
diff --git a/packages/happy-dom/src/html-serializer/HTMLSerializer.ts b/packages/happy-dom/src/html-serializer/HTMLSerializer.ts
index dede1814a..94ed8352e 100644
--- a/packages/happy-dom/src/html-serializer/HTMLSerializer.ts
+++ b/packages/happy-dom/src/html-serializer/HTMLSerializer.ts
@@ -10,6 +10,8 @@ import ShadowRoot from '../nodes/shadow-root/ShadowRoot.js';
 import HTMLElementConfig from '../config/HTMLElementConfig.js';
 import HTMLElementConfigContentModelEnum from '../config/HTMLElementConfigContentModelEnum.js';
 import XMLEncodeUtility from '../utilities/XMLEncodeUtility.js';
+import XMLSerializer from '../xml-serializer/XMLSerializer.js';
+import NamespaceURI from '../config/NamespaceURI.js';
 
 /**
  * Serializes a node into HTML.
@@ -60,6 +62,10 @@ export default class HTMLSerializer {
 	 * @returns Result.
 	 */
 	public serializeToString(root: Node): string {
+		const document = root[PropertySymbol.ownerDocument] || root;
+		if (document[PropertySymbol.contentType] !== 'text/html') {
+			return new XMLSerializer().serializeToString(root);
+		}
 		switch (root[PropertySymbol.nodeType]) {
 			case NodeTypeEnum.elementNode:
 				const element = <Element>root;
@@ -68,7 +74,10 @@ export default class HTMLSerializer {
 				const config = HTMLElementConfig[element[PropertySymbol.localName]];
 				const tagName = prefix ? `${prefix}:${localName}` : localName;
 
-				if (config?.contentModel === HTMLElementConfigContentModelEnum.noDescendants) {
+				if (
+					element[PropertySymbol.namespaceURI] === NamespaceURI.html &&
+					config?.contentModel === HTMLElementConfigContentModelEnum.noDescendants
+				) {
 					return `<${tagName}${this.getAttributes(element)}>`;
 				}
 
@@ -94,7 +103,7 @@ export default class HTMLSerializer {
 				}
 
 				const childNodes =
-					tagName === 'template'
+					element[PropertySymbol.namespaceURI] === NamespaceURI.html && tagName === 'template'
 						? (<DocumentFragment>(<HTMLTemplateElement>root).content)[PropertySymbol.nodeArray]
 						: (<DocumentFragment>root)[PropertySymbol.nodeArray];
 
diff --git a/packages/happy-dom/src/nodes/attr/Attr.ts b/packages/happy-dom/src/nodes/attr/Attr.ts
index eccdfa2a7..8a23511fc 100644
--- a/packages/happy-dom/src/nodes/attr/Attr.ts
+++ b/packages/happy-dom/src/nodes/attr/Attr.ts
@@ -21,6 +21,13 @@ export default class Attr extends Node implements Attr {
 	public [PropertySymbol.specified] = true;
 	public [PropertySymbol.ownerElement]: Element | null = null;
 
+	/**
+	 * @override
+	 */
+	public override get nodeName(): string {
+		return this[PropertySymbol.name];
+	}
+
 	/**
 	 * Returns specified.
 	 *
diff --git a/packages/happy-dom/src/nodes/comment/Comment.ts b/packages/happy-dom/src/nodes/comment/Comment.ts
index 87a4f8bc1..d671152d0 100644
--- a/packages/happy-dom/src/nodes/comment/Comment.ts
+++ b/packages/happy-dom/src/nodes/comment/Comment.ts
@@ -10,11 +10,9 @@ export default class Comment extends CharacterData {
 	public declare cloneNode: (deep?: boolean) => Comment;
 
 	/**
-	 * Node name.
-	 *
-	 * @returns Node name.
+	 * @override
 	 */
-	public get nodeName(): string {
+	public override get nodeName(): string {
 		return '#comment';
 	}
 
diff --git a/packages/happy-dom/src/nodes/document-fragment/DocumentFragment.ts b/packages/happy-dom/src/nodes/document-fragment/DocumentFragment.ts
index ed45c25e8..23477d786 100644
--- a/packages/happy-dom/src/nodes/document-fragment/DocumentFragment.ts
+++ b/packages/happy-dom/src/nodes/document-fragment/DocumentFragment.ts
@@ -93,6 +93,13 @@ export default class DocumentFragment extends Node {
 		}
 	}
 
+	/**
+	 * @override
+	 */
+	public override get nodeName(): string {
+		return '#document-fragment';
+	}
+
 	/**
 	 * Inserts a set of Node objects or DOMString objects after the last child of the ParentNode. DOMString objects are inserted as equivalent Text nodes.
 	 *
diff --git a/packages/happy-dom/src/nodes/document-type/DocumentType.ts b/packages/happy-dom/src/nodes/document-type/DocumentType.ts
index 8b7903159..e49576e44 100644
--- a/packages/happy-dom/src/nodes/document-type/DocumentType.ts
+++ b/packages/happy-dom/src/nodes/document-type/DocumentType.ts
@@ -40,12 +40,10 @@ export default class DocumentType extends Node {
 	}
 
 	/**
-	 * Node name.
-	 *
-	 * @returns Node name.
+	 * @override
 	 */
-	public get nodeName(): string {
-		return this.name;
+	public override get nodeName(): string {
+		return this[PropertySymbol.name];
 	}
 
 	/**
diff --git a/packages/happy-dom/src/nodes/document/Document.ts b/packages/happy-dom/src/nodes/document/Document.ts
index 7092f9bba..90bfc41eb 100644
--- a/packages/happy-dom/src/nodes/document/Document.ts
+++ b/packages/happy-dom/src/nodes/document/Document.ts
@@ -196,6 +196,13 @@ export default class Document extends Node {
 	public onpaste: (event: Event) => void = null;
 	public onbeforematch: (event: Event) => void = null;
 
+	/**
+	 * @override
+	 */
+	public override get nodeName(): string {
+		return '#document';
+	}
+
 	/**
 	 * Returns adopted style sheets.
 	 *
@@ -395,15 +402,6 @@ export default class Document extends Node {
 		]);
 	}
 
-	/**
-	 * Node name.
-	 *
-	 * @returns Node name.
-	 */
-	public get nodeName(): string {
-		return '#document';
-	}
-
 	/**
 	 * Returns <html> element.
 	 *
@@ -433,10 +431,8 @@ export default class Document extends Node {
 	 * @returns Element.
 	 */
 	public get body(): HTMLBodyElement {
-		const documentElement = this.documentElement;
-		return documentElement
-			? <HTMLBodyElement>ParentNodeUtility.getElementByTagName(documentElement, 'body')
-			: null;
+		// Returns null for XMLDocument.
+		return null;
 	}
 
 	/**
@@ -445,10 +441,8 @@ export default class Document extends Node {
 	 * @returns Element.
 	 */
 	public get head(): HTMLHeadElement {
-		const documentElement = this.documentElement;
-		return documentElement
-			? <HTMLHeadElement>ParentNodeUtility.getElementByTagName(documentElement, 'head')
-			: null;
+		// Returns null for XMLDocument.
+		return null;
 	}
 
 	/**
@@ -1072,16 +1066,30 @@ export default class Document extends Node {
 				svgElement[PropertySymbol.isValue] = options && options.is ? String(options.is) : null;
 
 				return svgElement;
+			case NamespaceURI.mathML:
+				const mathMLElement = NodeFactory.createNode<Element>(this, window.MathMLElement);
+
+				mathMLElement[PropertySymbol.tagName] = qualifiedName;
+				mathMLElement[PropertySymbol.localName] = localName;
+				mathMLElement[PropertySymbol.prefix] = prefix;
+				mathMLElement[PropertySymbol.namespaceURI] = namespaceURI;
+				mathMLElement[PropertySymbol.isValue] = options && options.is ? String(options.is) : null;
+
+				return mathMLElement;
 			case NamespaceURI.html:
 				// Custom HTML element
 				// If a polyfill is used, [PropertySymbol.registry] may be undefined
 				const customElementDefinition = window.customElements[PropertySymbol.registry]?.get(
 					options && options.is ? String(options.is) : qualifiedName
 				);
+				const tagName =
+					this[PropertySymbol.contentType] === 'text/html'
+						? StringUtility.asciiUpperCase(qualifiedName)
+						: qualifiedName;
 
 				if (customElementDefinition) {
 					const element = new customElementDefinition.elementClass();
-					element[PropertySymbol.tagName] = StringUtility.asciiUpperCase(qualifiedName);
+					element[PropertySymbol.tagName] = tagName;
 					element[PropertySymbol.localName] = localName;
 					element[PropertySymbol.prefix] = prefix;
 					element[PropertySymbol.namespaceURI] = namespaceURI;
@@ -1097,7 +1105,7 @@ export default class Document extends Node {
 				if (elementClass) {
 					const element = NodeFactory.createNode<Element>(this, elementClass);
 
-					element[PropertySymbol.tagName] = StringUtility.asciiUpperCase(qualifiedName);
+					element[PropertySymbol.tagName] = tagName;
 					element[PropertySymbol.localName] = localName;
 					element[PropertySymbol.prefix] = prefix;
 					element[PropertySymbol.namespaceURI] = namespaceURI;
@@ -1113,7 +1121,7 @@ export default class Document extends Node {
 
 				const unknownElement = NodeFactory.createNode<Element>(this, unknownElementClass);
 
-				unknownElement[PropertySymbol.tagName] = StringUtility.asciiUpperCase(qualifiedName);
+				unknownElement[PropertySymbol.tagName] = tagName;
 				unknownElement[PropertySymbol.localName] = localName;
 				unknownElement[PropertySymbol.prefix] = prefix;
 				unknownElement[PropertySymbol.namespaceURI] = namespaceURI;
diff --git a/packages/happy-dom/src/nodes/element/Element.ts b/packages/happy-dom/src/nodes/element/Element.ts
index 1c8990bbb..f1f9f2bdb 100644
--- a/packages/happy-dom/src/nodes/element/Element.ts
+++ b/packages/happy-dom/src/nodes/element/Element.ts
@@ -28,7 +28,6 @@ import MutationTypeEnum from '../../mutation-observer/MutationTypeEnum.js';
 import NamespaceURI from '../../config/NamespaceURI.js';
 import NodeList from '../node/NodeList.js';
 import CSSStyleDeclaration from '../../css/declaration/CSSStyleDeclaration.js';
-import NamedNodeMapProxyFactory from './NamedNodeMapProxyFactory.js';
 import NodeFactory from '../NodeFactory.js';
 import HTMLSerializer from '../../html-serializer/HTMLSerializer.js';
 import HTMLParser from '../../html-parser/HTMLParser.js';
@@ -95,7 +94,6 @@ export default class Element
 	public [PropertySymbol.scrollTop] = 0;
 	public [PropertySymbol.scrollLeft] = 0;
 	public [PropertySymbol.attributes] = new NamedNodeMap(this);
-	public [PropertySymbol.attributesProxy]: NamedNodeMap | null = null;
 	public [PropertySymbol.children]: HTMLCollection<Element> | null = null;
 	public [PropertySymbol.computedStyle]: CSSStyleDeclaration | null = null;
 	public declare [PropertySymbol.tagName]: string | null;
@@ -212,12 +210,7 @@ export default class Element
 	 * @returns Attributes.
 	 */
 	public get attributes(): NamedNodeMap {
-		if (!this[PropertySymbol.attributesProxy]) {
-			this[PropertySymbol.attributesProxy] = NamedNodeMapProxyFactory.createProxy(
-				this[PropertySymbol.attributes]
-			);
-		}
-		return this[PropertySymbol.attributesProxy];
+		return this[PropertySymbol.attributes];
 	}
 
 	/**
@@ -296,9 +289,7 @@ export default class Element
 	}
 
 	/**
-	 * Node name.
-	 *
-	 * @returns Node name.
+	 * @override
 	 */
 	public get nodeName(): string {
 		return this[PropertySymbol.tagName];
diff --git a/packages/happy-dom/src/nodes/element/NamedNodeMap.ts b/packages/happy-dom/src/nodes/element/NamedNodeMap.ts
index 9b3be26f5..6937aac21 100644
--- a/packages/happy-dom/src/nodes/element/NamedNodeMap.ts
+++ b/packages/happy-dom/src/nodes/element/NamedNodeMap.ts
@@ -5,6 +5,7 @@ import DOMExceptionNameEnum from '../../exception/DOMExceptionNameEnum.js';
 import Element from './Element.js';
 import NamespaceURI from '../../config/NamespaceURI.js';
 import StringUtility from '../../utilities/StringUtility.js';
+import ClassMethodBinder from '../../utilities/ClassMethodBinder.js';
 
 /**
  * Named Node Map.
@@ -29,6 +30,109 @@ export default class NamedNodeMap {
 	 */
 	constructor(ownerElement: Element) {
 		this[PropertySymbol.ownerElement] = ownerElement;
+
+		const methodBinder = new ClassMethodBinder(this, [NamedNodeMap]);
+
+		return new Proxy<NamedNodeMap>(this, {
+			get: (target, property) => {
+				if (property === 'length') {
+					return target[PropertySymbol.namedItems].size;
+				}
+				if (property in target || typeof property === 'symbol') {
+					methodBinder.bind(property);
+					return target[property];
+				}
+				const index = Number(property);
+				if (!isNaN(index)) {
+					return Array.from(target[PropertySymbol.namedItems].values())[index]?.[0];
+				}
+				return target.getNamedItem(<string>property) || undefined;
+			},
+			set(target, property, newValue): boolean {
+				methodBinder.bind(property);
+				if (typeof property === 'symbol') {
+					target[property] = newValue;
+					return true;
+				}
+				const index = Number(property);
+				if (isNaN(index)) {
+					target[property] = newValue;
+				}
+				return true;
+			},
+			deleteProperty(target, property): boolean {
+				if (typeof property === 'symbol') {
+					delete target[property];
+					return true;
+				}
+				const index = Number(property);
+				if (isNaN(index)) {
+					delete target[property];
+				}
+				return true;
+			},
+			ownKeys(target): string[] {
+				const keys = Array.from(target[PropertySymbol.namedItems].keys());
+				for (let i = 0, max = target[PropertySymbol.namedItems].size; i < max; i++) {
+					keys.push(String(i));
+				}
+				return keys;
+			},
+			has(target, property): boolean {
+				if (typeof property === 'symbol') {
+					return false;
+				}
+
+				if (property in target || target[PropertySymbol.namedItems].has(property)) {
+					return true;
+				}
+
+				const index = Number(property);
+
+				if (!isNaN(index) && index >= 0 && index < target[PropertySymbol.namedItems].size) {
+					return true;
+				}
+
+				return false;
+			},
+			defineProperty(target, property, descriptor): boolean {
+				methodBinder.preventBinding(property);
+
+				if (property in target) {
+					Object.defineProperty(target, property, descriptor);
+					return true;
+				}
+
+				return false;
+			},
+			getOwnPropertyDescriptor(target, property): PropertyDescriptor {
+				if (property in target || typeof property === 'symbol') {
+					return;
+				}
+
+				const index = Number(property);
+
+				if (!isNaN(index) && index >= 0 && index < target[PropertySymbol.namedItems].size) {
+					return {
+						value: Array.from(target[PropertySymbol.namedItems].values())[index][0],
+						writable: false,
+						enumerable: true,
+						configurable: true
+					};
+				}
+
+				const namedItems = target[PropertySymbol.namedItems].get(<string>property);
+
+				if (namedItems) {
+					return {
+						value: namedItems[0],
+						writable: false,
+						enumerable: true,
+						configurable: true
+					};
+				}
+			}
+		});
 	}
 
 	/**
diff --git a/packages/happy-dom/src/nodes/html-document/HTMLDocument.ts b/packages/happy-dom/src/nodes/html-document/HTMLDocument.ts
index f08b4fffc..dfc510fe5 100644
--- a/packages/happy-dom/src/nodes/html-document/HTMLDocument.ts
+++ b/packages/happy-dom/src/nodes/html-document/HTMLDocument.ts
@@ -2,6 +2,9 @@ import Document from '../document/Document.js';
 import * as PropertySymbol from '../../PropertySymbol.js';
 import Node from '../node/Node.js';
 import NodeTypeEnum from '../node/NodeTypeEnum.js';
+import HTMLBodyElement from '../html-body-element/HTMLBodyElement.js';
+import ParentNodeUtility from '../parent-node/ParentNodeUtility.js';
+import HTMLHeadElement from '../html-head-element/HTMLHeadElement.js';
 
 /**
  * Document.
@@ -24,6 +27,30 @@ export default class HTMLDocument extends Document {
 		documentElement.appendChild(bodyElement);
 	}
 
+	/**
+	 * Returns <body> element.
+	 *
+	 * @returns Element.
+	 */
+	public get body(): HTMLBodyElement {
+		const documentElement = this.documentElement;
+		return documentElement
+			? <HTMLBodyElement>ParentNodeUtility.getElementByTagName(documentElement, 'body')
+			: null;
+	}
+
+	/**
+	 * Returns <head> element.
+	 *
+	 * @returns Element.
+	 */
+	public get head(): HTMLHeadElement {
+		const documentElement = this.documentElement;
+		return documentElement
+			? <HTMLHeadElement>ParentNodeUtility.getElementByTagName(documentElement, 'head')
+			: null;
+	}
+
 	/**
 	 * @override
 	 */
diff --git a/packages/happy-dom/src/nodes/html-element/HTMLElementUtility.ts b/packages/happy-dom/src/nodes/html-element/HTMLElementUtility.ts
index 4d564aaf5..65e69d116 100644
--- a/packages/happy-dom/src/nodes/html-element/HTMLElementUtility.ts
+++ b/packages/happy-dom/src/nodes/html-element/HTMLElementUtility.ts
@@ -1,6 +1,7 @@
 import FocusEvent from '../../event/events/FocusEvent.js';
 import * as PropertySymbol from '../../PropertySymbol.js';
 import HTMLElement from '../html-element/HTMLElement.js';
+import MathMLElement from '../math-ml-element/MathMLElement.js';
 import SVGElement from '../svg-element/SVGElement.js';
 
 /**
@@ -12,7 +13,7 @@ export default class HTMLElementUtility {
 	 *
 	 * @param element Element.
 	 */
-	public static blur(element: HTMLElement | SVGElement): void {
+	public static blur(element: HTMLElement | SVGElement | MathMLElement): void {
 		const target = element[PropertySymbol.proxy] || element;
 		const document = target[PropertySymbol.ownerDocument];
 
@@ -49,7 +50,7 @@ export default class HTMLElementUtility {
 	 *
 	 * @param element Element.
 	 */
-	public static focus(element: HTMLElement | SVGElement): void {
+	public static focus(element: HTMLElement | SVGElement | MathMLElement): void {
 		const target = element[PropertySymbol.proxy] || element;
 		const document = target[PropertySymbol.ownerDocument];
 
diff --git a/packages/happy-dom/src/nodes/math-ml-element/MathMLElement.ts b/packages/happy-dom/src/nodes/math-ml-element/MathMLElement.ts
new file mode 100644
index 000000000..f0d92b019
--- /dev/null
+++ b/packages/happy-dom/src/nodes/math-ml-element/MathMLElement.ts
@@ -0,0 +1,81 @@
+import CSSStyleDeclaration from '../../css/declaration/CSSStyleDeclaration.js';
+import * as PropertySymbol from '../../PropertySymbol.js';
+import Element from '../element/Element.js';
+import HTMLElementUtility from '../html-element/HTMLElementUtility.js';
+import DOMStringMap from '../../dom/DOMStringMap.js';
+
+/**
+ * Math ML Element.
+ *
+ * Reference:
+ * https://developer.mozilla.org/en-US/docs/Web/API/MathMLElement.
+ */
+export default class MathMLElement extends Element {
+	// Internal properties
+	public [PropertySymbol.style]: CSSStyleDeclaration | null = null;
+
+	// Private properties
+	#dataset: DOMStringMap | null = null;
+
+	/**
+	 * Returns data set.
+	 *
+	 * @returns Data set.
+	 */
+	public get dataset(): DOMStringMap {
+		return (this.#dataset ??= new DOMStringMap(PropertySymbol.illegalConstructor, this));
+	}
+
+	/**
+	 * Returns style.
+	 *
+	 * @returns Style.
+	 */
+	public get style(): CSSStyleDeclaration {
+		if (!this[PropertySymbol.style]) {
+			this[PropertySymbol.style] = new CSSStyleDeclaration(
+				PropertySymbol.illegalConstructor,
+				this[PropertySymbol.window],
+				{ element: this }
+			);
+		}
+		return this[PropertySymbol.style];
+	}
+
+	/**
+	 * Returns tab index.
+	 *
+	 * @returns Tab index.
+	 */
+	public get tabIndex(): number {
+		const tabIndex = this.getAttribute('tabindex');
+		return tabIndex !== null ? Number(tabIndex) : -1;
+	}
+
+	/**
+	 * Returns tab index.
+	 *
+	 * @param tabIndex Tab index.
+	 */
+	public set tabIndex(tabIndex: number) {
+		if (tabIndex === -1) {
+			this.removeAttribute('tabindex');
+		} else {
+			this.setAttribute('tabindex', String(tabIndex));
+		}
+	}
+
+	/**
+	 * Triggers a blur event.
+	 */
+	public blur(): void {
+		HTMLElementUtility.blur(this);
+	}
+
+	/**
+	 * Triggers a focus event.
+	 */
+	public focus(): void {
+		HTMLElementUtility.focus(this);
+	}
+}
diff --git a/packages/happy-dom/src/nodes/node/NodeUtility.ts b/packages/happy-dom/src/nodes/node/NodeUtility.ts
index 8b5a2c323..4825aea6b 100644
--- a/packages/happy-dom/src/nodes/node/NodeUtility.ts
+++ b/packages/happy-dom/src/nodes/node/NodeUtility.ts
@@ -77,8 +77,8 @@ export default class NodeUtility {
 			parent = parent[PropertySymbol.parentNode]
 				? parent[PropertySymbol.parentNode]
 				: includeShadowRoots && (<ShadowRoot>parent).host
-					? (<ShadowRoot>parent).host
-					: null;
+				? (<ShadowRoot>parent).host
+				: null;
 		}
 
 		return false;
@@ -139,7 +139,38 @@ export default class NodeUtility {
 	}
 
 	/**
-	 * Returns boolean indicating if nodeB is following nodeA in the document tree.
+	 * Returns preceding node.
+	 *
+	 * Based on:
+	 * https://github.com/jsdom/js-symbol-tree/blob/master/lib/SymbolTree.js#L185
+	 *
+	 * @param node Node.
+	 * @param [root] Root.
+	 * @returns Following node.
+	 */
+	public static preceding(node: Node, root?: Node): Node {
+		if (node === root) {
+			return null;
+		}
+
+		const previousSibling = node.previousSibling;
+
+		if (previousSibling) {
+			let lastChild;
+			let current = node;
+
+			while ((lastChild = node.lastChild)) {
+				current = lastChild;
+			}
+
+			return current;
+		}
+
+		return node[PropertySymbol.parentNode];
+	}
+
+	/**
+	 * Returns following node.
 	 *
 	 * Based on:
 	 * https://github.com/jsdom/js-symbol-tree/blob/master/lib/SymbolTree.js#L220
diff --git a/packages/happy-dom/src/nodes/processing-instruction/ProcessingInstruction.ts b/packages/happy-dom/src/nodes/processing-instruction/ProcessingInstruction.ts
index 09321f0af..c44492b31 100644
--- a/packages/happy-dom/src/nodes/processing-instruction/ProcessingInstruction.ts
+++ b/packages/happy-dom/src/nodes/processing-instruction/ProcessingInstruction.ts
@@ -11,6 +11,13 @@ export default class ProcessingInstruction extends CharacterData {
 	public [PropertySymbol.nodeType] = NodeTypeEnum.processingInstructionNode;
 	public [PropertySymbol.target]: string;
 
+	/**
+	 * @override
+	 */
+	public override get nodeName(): string {
+		return this[PropertySymbol.target];
+	}
+
 	/**
 	 * Returns target.
 	 *
diff --git a/packages/happy-dom/src/nodes/text/Text.ts b/packages/happy-dom/src/nodes/text/Text.ts
index 84d0ea4bd..e747c923e 100644
--- a/packages/happy-dom/src/nodes/text/Text.ts
+++ b/packages/happy-dom/src/nodes/text/Text.ts
@@ -15,11 +15,9 @@ export default class Text extends CharacterData {
 	public override [PropertySymbol.styleNode]: HTMLStyleElement | null = null;
 
 	/**
-	 * Node name.
-	 *
-	 * @returns Node name.
+	 * @override
 	 */
-	public get nodeName(): string {
+	public override get nodeName(): string {
 		return '#text';
 	}
 
diff --git a/packages/happy-dom/src/tree-walker/NodeFilterUtility.ts b/packages/happy-dom/src/tree-walker/NodeFilterUtility.ts
new file mode 100644
index 000000000..5eec0d9c6
--- /dev/null
+++ b/packages/happy-dom/src/tree-walker/NodeFilterUtility.ts
@@ -0,0 +1,44 @@
+import NodeFilterMask from './NodeFilterMask.js';
+import * as PropertySymbol from '../PropertySymbol.js';
+import Node from '../nodes/node/Node.js';
+import NodeFilter from './NodeFilter.js';
+import INodeFilter from './INodeFilter.js';
+
+/**
+ *
+ */
+export default class NodeFilterUtility {
+	/**
+	 * Filters a node.
+	 *
+	 * Based on solution:
+	 * https://gist.github.com/shawndumas/1132009.
+	 *
+	 * @param node Node.
+	 * @param options Filter options.
+	 * @param options.whatToShow What to show.
+	 * @param options.filter Filter.
+	 * @returns Filter result.
+	 */
+	public static filterNode(
+		node: Node,
+		options: {
+			whatToShow: number;
+			filter: INodeFilter | null;
+		}
+	): number {
+		const mask = NodeFilterMask[node[PropertySymbol.nodeType]];
+
+		if (mask && (options.whatToShow & mask) == 0) {
+			return NodeFilter.FILTER_SKIP;
+		}
+		if (typeof options.filter === 'function') {
+			return options.filter(node);
+		}
+		if (options.filter) {
+			return options.filter.acceptNode(node);
+		}
+
+		return NodeFilter.FILTER_ACCEPT;
+	}
+}
diff --git a/packages/happy-dom/src/tree-walker/NodeIterator.ts b/packages/happy-dom/src/tree-walker/NodeIterator.ts
index 50eadbb8b..6188e81dc 100644
--- a/packages/happy-dom/src/tree-walker/NodeIterator.ts
+++ b/packages/happy-dom/src/tree-walker/NodeIterator.ts
@@ -1,8 +1,8 @@
 import INodeFilter from './INodeFilter.js';
-import TreeWalker from './TreeWalker.js';
 import Node from '../nodes/node/Node.js';
-import * as PropertySymbol from '../PropertySymbol.js';
 import NodeFilter from './NodeFilter.js';
+import NodeFilterUtility from './NodeFilterUtility.js';
+import * as PropertySymbol from '../PropertySymbol.js';
 
 /**
  * The NodeIterator object represents the nodes of a document subtree and a position within them.
@@ -11,11 +11,17 @@ import NodeFilter from './NodeFilter.js';
  * https://developer.mozilla.org/en-US/docs/Web/API/NodeIterator
  */
 export default class NodeIterator {
-	#root: Node = null;
-	#whatToShow = -1;
-	#filter: INodeFilter = null;
-	#walker: TreeWalker;
-	#atRoot = true;
+	#root: Node;
+	#referenceNode: Node;
+	#filterOptions: {
+		whatToShow: number;
+		filter: INodeFilter | null;
+	} = {
+		whatToShow: -1,
+		filter: null
+	};
+	#index = -1;
+	#pointerBeforeReferenceNode = true;
 
 	/**
 	 * Constructor.
@@ -26,9 +32,9 @@ export default class NodeIterator {
 	 */
 	constructor(root: Node, whatToShow = -1, filter: INodeFilter = null) {
 		this.#root = root;
-		this.#whatToShow = whatToShow;
-		this.#filter = filter;
-		this.#walker = new TreeWalker(root, whatToShow, filter);
+		this.#referenceNode = root;
+		this.#filterOptions.whatToShow = whatToShow;
+		this.#filterOptions.filter = filter;
 	}
 
 	/**
@@ -46,7 +52,7 @@ export default class NodeIterator {
 	 * @returns What to show.
 	 */
 	public get whatToShow(): number {
-		return this.#whatToShow;
+		return this.#filterOptions.whatToShow;
 	}
 
 	/**
@@ -55,7 +61,25 @@ export default class NodeIterator {
 	 * @returns Filter.
 	 */
 	public get filter(): INodeFilter {
-		return this.#filter;
+		return this.#filterOptions.filter;
+	}
+
+	/**
+	 * Returns reference node.
+	 *
+	 * @returns Reference node.
+	 */
+	public get referenceNode(): Node | null {
+		return this.#referenceNode || null;
+	}
+
+	/**
+	 * Returns pointer before reference node.
+	 *
+	 * @returns Pointer before reference node.
+	 */
+	public get pointerBeforeReferenceNode(): boolean {
+		return this.#pointerBeforeReferenceNode;
 	}
 
 	/**
@@ -64,14 +88,27 @@ export default class NodeIterator {
 	 * @returns Current node.
 	 */
 	public nextNode(): Node {
-		if (this.#atRoot) {
-			this.#atRoot = false;
-			if (this.#walker[PropertySymbol.filterNode](this.#root) !== NodeFilter.FILTER_ACCEPT) {
-				return this.#walker.nextNode();
+		const nodes = this.#getNodes(this.#root);
+
+		// If the current node has been removed we need to step back one node.
+		if (this.#index !== -1 && this.#referenceNode !== nodes[this.#index]) {
+			this.#index--;
+		}
+
+		this.#pointerBeforeReferenceNode = false;
+
+		while (this.#index < nodes.length - 1) {
+			this.#index++;
+
+			const node = nodes[this.#index];
+
+			if (NodeFilterUtility.filterNode(node, this.#filterOptions) === NodeFilter.FILTER_ACCEPT) {
+				this.#referenceNode = node;
+				return node;
 			}
-			return this.#root;
 		}
-		return this.#walker.nextNode();
+
+		return null;
 	}
 
 	/**
@@ -80,6 +117,51 @@ export default class NodeIterator {
 	 * @returns Current node.
 	 */
 	public previousNode(): Node {
-		return this.#walker.previousNode();
+		const nodes = this.#getNodes(this.#root);
+
+		if (this.#index !== -1 && this.#referenceNode !== nodes[this.#index]) {
+			this.#index++;
+		}
+
+		this.#pointerBeforeReferenceNode = true;
+
+		while (this.#index > 0) {
+			this.#index--;
+
+			const node = nodes[this.#index];
+
+			if (NodeFilterUtility.filterNode(node, this.#filterOptions) === NodeFilter.FILTER_ACCEPT) {
+				this.#referenceNode = node;
+				return node;
+			}
+		}
+
+		return null;
+	}
+
+	/**
+	 * This is a legacy method, and no longer has any effect.
+	 *
+	 * Previously it served to mark a NodeIterator as disposed, so it could be reclaimed by garbage collection.
+	 */
+	public detach(): void {
+		// Do nothing as per the spec.
+	}
+
+	/**
+	 * Returns the nodes of the iterator.
+	 *
+	 * @param root Root.
+	 * @param [nodes] Nodes.
+	 */
+	#getNodes(root: Node, nodes?: Node[]): Node[] {
+		nodes = nodes || [root];
+
+		for (const node of root[PropertySymbol.nodeArray]) {
+			nodes.push(node);
+			this.#getNodes(node, nodes);
+		}
+
+		return nodes;
 	}
 }
diff --git a/packages/happy-dom/src/tree-walker/TreeWalker.ts b/packages/happy-dom/src/tree-walker/TreeWalker.ts
index f4a34b733..46126d983 100644
--- a/packages/happy-dom/src/tree-walker/TreeWalker.ts
+++ b/packages/happy-dom/src/tree-walker/TreeWalker.ts
@@ -1,18 +1,26 @@
 import Node from '../nodes/node/Node.js';
 import * as PropertySymbol from '../PropertySymbol.js';
 import INodeFilter from './INodeFilter.js';
-import NodeFilterMask from './NodeFilterMask.js';
 import DOMException from '../exception/DOMException.js';
 import NodeFilter from './NodeFilter.js';
+import NodeFilterUtility from './NodeFilterUtility.js';
 
 /**
  * The TreeWalker object represents the nodes of a document subtree and a position within them.
  */
 export default class TreeWalker {
-	public root: Node = null;
-	public whatToShow = -1;
-	public filter: INodeFilter = null;
-	public currentNode: Node = null;
+	// Internal properties.
+	public [PropertySymbol.currentNode]: Node | null = null;
+
+	// Private properties.
+	#root: Node;
+	#filterOptions: {
+		whatToShow: number;
+		filter: INodeFilter | null;
+	} = {
+		whatToShow: -1,
+		filter: null
+	};
 
 	/**
 	 * Constructor.
@@ -26,10 +34,46 @@ export default class TreeWalker {
 			throw new DOMException('Parameter 1 was not of type Node.');
 		}
 
-		this.root = root;
-		this.whatToShow = whatToShow;
-		this.filter = filter;
-		this.currentNode = root;
+		this.#root = root;
+		this.#filterOptions.whatToShow = whatToShow;
+		this.#filterOptions.filter = filter;
+		this[PropertySymbol.currentNode] = root;
+	}
+
+	/**
+	 * Returns root.
+	 *
+	 * @returns Root.
+	 */
+	public get root(): Node | null {
+		return this.#root;
+	}
+
+	/**
+	 * Returns what to show.
+	 *
+	 * @returns What to show.
+	 */
+	public get whatToShow(): number {
+		return this.#filterOptions.whatToShow;
+	}
+
+	/**
+	 * Returns filter.
+	 *
+	 * @returns Filter.
+	 */
+	public get filter(): INodeFilter {
+		return this.#filterOptions.filter;
+	}
+
+	/**
+	 * Returns current node.
+	 *
+	 * @returns Current node.
+	 */
+	public get currentNode(): Node {
+		return this[PropertySymbol.currentNode];
 	}
 
 	/**
@@ -40,7 +84,8 @@ export default class TreeWalker {
 	public nextNode(): Node {
 		if (!this.firstChild()) {
 			while (!this.nextSibling() && this.parentNode()) {}
-			this.currentNode = this.currentNode === this.root ? null : this.currentNode || null;
+			this[PropertySymbol.currentNode] =
+				this.currentNode === this.root ? null : this.currentNode || null;
 		}
 		return this.currentNode;
 	}
@@ -52,7 +97,8 @@ export default class TreeWalker {
 	 */
 	public previousNode(): Node {
 		while (!this.previousSibling() && this.parentNode()) {}
-		this.currentNode = this.currentNode === this.root ? null : this.currentNode || null;
+		this[PropertySymbol.currentNode] =
+			this.currentNode === this.root ? null : this.currentNode || null;
 		return this.currentNode;
 	}
 
@@ -67,16 +113,19 @@ export default class TreeWalker {
 			this.currentNode &&
 			this.currentNode[PropertySymbol.parentNode]
 		) {
-			this.currentNode = this.currentNode[PropertySymbol.parentNode];
+			this[PropertySymbol.currentNode] = this.currentNode[PropertySymbol.parentNode];
 
-			if (this[PropertySymbol.filterNode](this.currentNode) === NodeFilter.FILTER_ACCEPT) {
+			if (
+				NodeFilterUtility.filterNode(this.currentNode, this.#filterOptions) ===
+				NodeFilter.FILTER_ACCEPT
+			) {
 				return this.currentNode;
 			}
 
 			this.parentNode();
 		}
 
-		this.currentNode = null;
+		this[PropertySymbol.currentNode] = null;
 
 		return null;
 	}
@@ -90,9 +139,12 @@ export default class TreeWalker {
 		const childNodes = this.currentNode ? (<Node>this.currentNode)[PropertySymbol.nodeArray] : [];
 
 		if (childNodes.length > 0) {
-			this.currentNode = childNodes[0];
+			this[PropertySymbol.currentNode] = childNodes[0];
 
-			if (this[PropertySymbol.filterNode](this.currentNode) === NodeFilter.FILTER_ACCEPT) {
+			if (
+				NodeFilterUtility.filterNode(this.currentNode, this.#filterOptions) ===
+				NodeFilter.FILTER_ACCEPT
+			) {
 				return this.currentNode;
 			}
 
@@ -111,9 +163,12 @@ export default class TreeWalker {
 		const childNodes = this.currentNode ? (<Node>this.currentNode)[PropertySymbol.nodeArray] : [];
 
 		if (childNodes.length > 0) {
-			this.currentNode = childNodes[childNodes.length - 1];
+			this[PropertySymbol.currentNode] = childNodes[childNodes.length - 1];
 
-			if (this[PropertySymbol.filterNode](this.currentNode) === NodeFilter.FILTER_ACCEPT) {
+			if (
+				NodeFilterUtility.filterNode(this.currentNode, this.#filterOptions) ===
+				NodeFilter.FILTER_ACCEPT
+			) {
 				return this.currentNode;
 			}
 
@@ -140,9 +195,12 @@ export default class TreeWalker {
 			const index = siblings.indexOf(this.currentNode);
 
 			if (index > 0) {
-				this.currentNode = siblings[index - 1];
+				this[PropertySymbol.currentNode] = siblings[index - 1];
 
-				if (this[PropertySymbol.filterNode](this.currentNode) === NodeFilter.FILTER_ACCEPT) {
+				if (
+					NodeFilterUtility.filterNode(this.currentNode, this.#filterOptions) ===
+					NodeFilter.FILTER_ACCEPT
+				) {
 					return this.currentNode;
 				}
 
@@ -170,9 +228,12 @@ export default class TreeWalker {
 			const index = siblings.indexOf(this.currentNode);
 
 			if (index + 1 < siblings.length) {
-				this.currentNode = siblings[index + 1];
+				this[PropertySymbol.currentNode] = siblings[index + 1];
 
-				if (this[PropertySymbol.filterNode](this.currentNode) === NodeFilter.FILTER_ACCEPT) {
+				if (
+					NodeFilterUtility.filterNode(this.currentNode, this.#filterOptions) ===
+					NodeFilter.FILTER_ACCEPT
+				) {
 					return this.currentNode;
 				}
 
@@ -182,29 +243,4 @@ export default class TreeWalker {
 
 		return null;
 	}
-
-	/**
-	 * Filters a node.
-	 *
-	 * Based on solution:
-	 * https://gist.github.com/shawndumas/1132009.
-	 *
-	 * @param node Node.
-	 * @returns Child nodes.
-	 */
-	public [PropertySymbol.filterNode](node: Node): number {
-		const mask = NodeFilterMask[node.nodeType];
-
-		if (mask && (this.whatToShow & mask) == 0) {
-			return NodeFilter.FILTER_SKIP;
-		}
-		if (typeof this.filter === 'function') {
-			return this.filter(node);
-		}
-		if (this.filter) {
-			return this.filter.acceptNode(node);
-		}
-
-		return NodeFilter.FILTER_ACCEPT;
-	}
 }
diff --git a/packages/happy-dom/src/window/BrowserWindow.ts b/packages/happy-dom/src/window/BrowserWindow.ts
index e0722fd22..e73ad8589 100644
--- a/packages/happy-dom/src/window/BrowserWindow.ts
+++ b/packages/happy-dom/src/window/BrowserWindow.ts
@@ -307,6 +307,7 @@ import DOMPoint from '../dom/DOMPoint.js';
 import SVGAnimatedLengthList from '../svg/SVGAnimatedLengthList.js';
 import CustomElementReactionStack from '../custom-element/CustomElementReactionStack.js';
 import IScrollToOptions from './IScrollToOptions.js';
+import MathMLElement from '../nodes/math-ml-element/MathMLElement.js';
 
 const TIMER = {
 	setTimeout: globalThis.setTimeout.bind(globalThis),
@@ -439,6 +440,9 @@ export default class BrowserWindow extends EventTarget implements INodeJSGlobal
 	public readonly HTMLBodyElement = HTMLBodyElement;
 	public readonly HTMLAreaElement = HTMLAreaElement;
 
+	// MathML Element classes
+	public readonly MathMLElement = MathMLElement;
+
 	// SVG Element classes
 	public readonly SVGSVGElement = SVGSVGElement;
 	public readonly SVGAnimateElement = SVGAnimateElement;
@@ -573,6 +577,7 @@ export default class BrowserWindow extends EventTarget implements INodeJSGlobal
 	public readonly UserProximityEvent = Event;
 	public readonly WebGLContextEvent = Event;
 	public readonly TextEvent = Event;
+	public readonly NamedNodeMap = NamedNodeMap;
 
 	// Other classes that has to be bound to the Window context (populated by WindowContextClassExtender)
 	public declare readonly NodeIterator: typeof NodeIterator;
@@ -606,7 +611,6 @@ export default class BrowserWindow extends EventTarget implements INodeJSGlobal
 	public declare readonly MediaStream: typeof MediaStream;
 	public declare readonly MediaStreamTrack: typeof MediaStreamTrack;
 	public declare readonly CanvasCaptureMediaStreamTrack: typeof CanvasCaptureMediaStreamTrack;
-	public declare readonly NamedNodeMap: typeof NamedNodeMap;
 	public declare readonly TextTrack: typeof TextTrack;
 	public declare readonly TextTrackList: typeof TextTrackList;
 	public declare readonly TextTrackCue: typeof TextTrackCue;
diff --git a/packages/happy-dom/src/window/WindowContextClassExtender.ts b/packages/happy-dom/src/window/WindowContextClassExtender.ts
index eda66d859..d55e6cefd 100644
--- a/packages/happy-dom/src/window/WindowContextClassExtender.ts
+++ b/packages/happy-dom/src/window/WindowContextClassExtender.ts
@@ -45,7 +45,6 @@ import FileReaderImplementation from '../file/FileReader.js';
 import MediaStreamImplementation from '../nodes/html-media-element/MediaStream.js';
 import MediaStreamTrackImplementation from '../nodes/html-media-element/MediaStreamTrack.js';
 import CanvasCaptureMediaStreamTrackImplementation from '../nodes/html-canvas-element/CanvasCaptureMediaStreamTrack.js';
-import NamedNodeMapImplementation from '../nodes/element/NamedNodeMap.js';
 
 /**
  * Extends classes with a "window" property, so that they internally can access it's Window context.
@@ -284,11 +283,6 @@ export default class WindowContextClassExtender {
 		(<typeof CanvasCaptureMediaStreamTrack>window.CanvasCaptureMediaStreamTrack) =
 			CanvasCaptureMediaStreamTrack;
 
-		// NamedNodeMap
-		class NamedNodeMap extends NamedNodeMapImplementation {}
-		NamedNodeMap.prototype[PropertySymbol.window] = window;
-		(<typeof NamedNodeMap>window.NamedNodeMap) = NamedNodeMap;
-
 		/* eslint-enable jsdoc/require-jsdoc */
 	}
 }
diff --git a/packages/happy-dom/test/dom-parser/DOMParser.test.ts b/packages/happy-dom/test/dom-parser/DOMParser.test.ts
index 937f81b48..9b5ee2e96 100644
--- a/packages/happy-dom/test/dom-parser/DOMParser.test.ts
+++ b/packages/happy-dom/test/dom-parser/DOMParser.test.ts
@@ -142,7 +142,8 @@ describe('DOMParser', () => {
                 `,
 				'application/xml'
 			);
-			expect(new HTMLSerializer().serializeToString(newDocument)).toBe(`<breakfast_menu>
+			expect(new XMLSerializer().serializeToString(newDocument))
+				.toBe(`<?xml version="1.0" encoding="UTF-8"?><breakfast_menu>
                     <food>
                         <name>Belgian Waffles</name>
                         <price>$5.95</price>
diff --git a/packages/happy-dom/test/html-parser/HTMLParser.test.ts b/packages/happy-dom/test/html-parser/HTMLParser.test.ts
index a260b1975..7b28d4304 100644
--- a/packages/happy-dom/test/html-parser/HTMLParser.test.ts
+++ b/packages/happy-dom/test/html-parser/HTMLParser.test.ts
@@ -13,6 +13,11 @@ import CustomElement from '../CustomElement.js';
 import HTMLHtmlElement from '../../src/nodes/html-html-element/HTMLHtmlElement.js';
 import XMLSerializer from '../../src/xml-serializer/XMLSerializer.js';
 import TreeWalkerHTML from '../tree-walker/data/TreeWalkerHTML.js';
+import MathMLElementConfig from '../../src/config/MathMLElementConfig.js';
+import SVGSVGElement from '../../src/nodes/svg-svg-element/SVGSVGElement.js';
+import HTMLElementConfig from '../../src/config/HTMLElementConfig.js';
+import SVGElementConfig from '../../src/config/SVGElementConfig.js';
+import HTMLElementConfigContentModelEnum from '../../src/config/HTMLElementConfigContentModelEnum.js';
 
 describe('HTMLParser', () => {
 	let window: Window;
@@ -731,6 +736,143 @@ describe('HTMLParser', () => {
 			);
 		});
 
+		it('Parses malformed SVG elements with style and comments.', () => {
+			const result = new HTMLParser(window).parse(
+				`<svg><annotation-xml><foreignobject><style><!--</style><p id=\"--><img src='x' onerror='alert(1)'>\">`,
+				document.implementation.createHTMLDocument()
+			);
+
+			expect(new HTMLSerializer().serializeToString(result)).toBe(
+				`<html><head></head><body><svg><annotation-xml><foreignObject><style><!--</style><p id="--><img src='x' onerror='alert(1)'>"></p></foreignObject></annotation-xml></svg></body></html>`
+			);
+		});
+
+		it('Children of SVGForeignObjectElement should escape the SVG namespace', () => {
+			const result = new HTMLParser(window).parse(
+				`<svg><foreignobject><div><style></style></div><svg></foreignobject></svg>`,
+				document.implementation.createHTMLDocument()
+			);
+
+			expect(new HTMLSerializer().serializeToString(result)).toBe(
+				`<html><head></head><body><svg><foreignObject><div><style></style></div><svg></svg></foreignObject></svg></body></html>`
+			);
+
+			expect((<HTMLElement>result.querySelector('div')).namespaceURI).toBe(NamespaceURI.html);
+			expect((<HTMLElement>result.querySelector('style')).namespaceURI).toBe(NamespaceURI.html);
+			expect((<SVGSVGElement>result.querySelector('svg')).namespaceURI).toBe(NamespaceURI.svg);
+		});
+
+		it('It handles SVG element with unicode characters in tag name.', () => {
+			const result = new HTMLParser(window).parse(
+				`<svg><blocKquote>foo</blocKquote>`,
+				document.implementation.createHTMLDocument()
+			);
+
+			expect(new HTMLSerializer().serializeToString(result)).toBe(
+				`<html><head></head><body><svg></svg><blockquote>foo</blockquote></body></html>`
+			);
+		});
+
+		it('Handles HTML elements that should escape the SVG namespace.', () => {
+			for (const tag of Object.keys(HTMLElementConfig)) {
+				const config = HTMLElementConfig[tag];
+				if (config.escapesSVGNamespace) {
+					const result = new HTMLParser(window).parse(`<svg><${tag}></${tag}>Test</svg>`);
+					if (tag === 'body' || tag === 'head' || tag === 'html') {
+						expect(new HTMLSerializer().serializeToString(result)).toBe(`<svg></svg>Test`);
+					} else if (config.contentModel === HTMLElementConfigContentModelEnum.noDescendants) {
+						expect(new HTMLSerializer().serializeToString(result)).toBe(`<svg></svg><${tag}>Test`);
+					} else {
+						expect(new HTMLSerializer().serializeToString(result)).toBe(
+							`<svg></svg><${tag}></${tag}>Test`
+						);
+					}
+				} else {
+					const result = new HTMLParser(window).parse(`<svg><${tag}></${tag}>Test</svg>`);
+					expect(new HTMLSerializer().serializeToString(result)).toBe(
+						`<svg><${tag}></${tag}>Test</svg>`
+					);
+				}
+			}
+		});
+
+		it('Parses MathML elements.', () => {
+			const result = new HTMLParser(window).parse(
+				`<div>
+                    <math>
+                        <mi>
+                            <mglyph>x</mglyph>
+                        </mi>
+                    </math>
+                </div>`
+			);
+
+			expect(new HTMLSerializer().serializeToString(result)).toBe(
+				`<div>
+                    <math>
+                        <mi>
+                            <mglyph>x</mglyph>
+                        </mi>
+                    </math>
+                </div>`
+			);
+
+			expect(result.children[0].children[0]).toBeInstanceOf(window.MathMLElement);
+			expect(result.children[0].children[0].namespaceURI).toBe(NamespaceURI.mathML);
+			expect(result.children[0].children[0].children[0]).toBeInstanceOf(window.MathMLElement);
+			expect(result.children[0].children[0].children[0].namespaceURI).toBe(NamespaceURI.mathML);
+			expect(result.children[0].children[0].children[0].children[0]).toBeInstanceOf(
+				window.MathMLElement
+			);
+			expect(result.children[0].children[0].children[0].children[0].namespaceURI).toBe(
+				NamespaceURI.mathML
+			);
+			expect(result.children[0].children[0].children[0].children[0].textContent).toBe('x');
+		});
+
+		it("Escapes the MathML namespace in <mi> elements if it isn't a known Math ML element", () => {
+			const result = new HTMLParser(window).parse(
+				`<div>
+                    <math>
+                        <mi>
+                            ${Object.keys(MathMLElementConfig)
+															.map((key) => `<${key}></${key}>`)
+															.join('')}
+                        </mi>
+                        <mi>
+                            <svg></svg>
+                            <canvas></canvas>
+                        </mi>
+                    </math>
+                </div>`
+			);
+
+			const mi1 = result.children[0].children[0].children[0];
+
+			expect(mi1.children.length).toBe(Object.keys(MathMLElementConfig).length);
+
+			for (const child of mi1.children) {
+				expect(child.namespaceURI).toBe(NamespaceURI.mathML);
+			}
+
+			const mi2 = result.children[0].children[0].children[1];
+
+			expect(mi2.children.length).toBe(2);
+			expect(mi2.children[0].namespaceURI).toBe(NamespaceURI.svg);
+			expect(mi2.children[1].namespaceURI).toBe(NamespaceURI.html);
+		});
+
+		it('Parses malformed MathML elements.', () => {
+			const result = new HTMLParser(window).parse(
+				`<form><math><mi><mglyph></form><form>`,
+				document.implementation.createHTMLDocument()
+			);
+
+			expect(new HTMLSerializer().serializeToString(result)).toBe(
+				`<html><head></head><body><form><math><mi><mglyph><form></form></mglyph></mi></math></form></body></html>`
+			);
+		});
+
 		it('Parses childless elements with start and end tag names in different case', () => {
 			const result = new HTMLParser(window).parse(
 				`
diff --git a/packages/happy-dom/test/nodes/document/Document.test.ts b/packages/happy-dom/test/nodes/document/Document.test.ts
index b59c27e35..9fbab7209 100644
--- a/packages/happy-dom/test/nodes/document/Document.test.ts
+++ b/packages/happy-dom/test/nodes/document/Document.test.ts
@@ -1130,6 +1130,18 @@ describe('Document', () => {
 			expect(element).toBeInstanceOf(SVGElement);
 			expect(element.constructor.name).toBe('SVGElement');
 		});
+
+		it('Returns MathMLElement when namespace is MathML.', () => {
+			const element = document.createElementNS(NamespaceURI.mathML, 'math');
+			expect(element.tagName).toBe('math');
+			expect(element.localName).toBe('math');
+			expect(element).toBeInstanceOf(window.MathMLElement);
+
+			const element2 = document.createElementNS(NamespaceURI.mathML, 'mglyph');
+			expect(element2.tagName).toBe('mglyph');
+			expect(element2.localName).toBe('mglyph');
+			expect(element2).toBeInstanceOf(window.MathMLElement);
+		});
 	});
 
 	describe('createAttribute()', () => {
diff --git a/packages/happy-dom/test/nodes/element/Element.test.ts b/packages/happy-dom/test/nodes/element/Element.test.ts
index d6fc95481..b4a631c42 100644
--- a/packages/happy-dom/test/nodes/element/Element.test.ts
+++ b/packages/happy-dom/test/nodes/element/Element.test.ts
@@ -349,7 +349,11 @@ describe('Element', () => {
 	});
 
 	describe('get attributes()', () => {
-		it('Returns all attributes as an object.', () => {
+		it('Returns a NamedNodeMap object.', () => {
+			expect(element.attributes).toBeInstanceOf(window.NamedNodeMap);
+		});
+
+		it('Returns all attributes.', () => {
 			element.setAttribute('key1', 'value1');
 			element.setAttribute('key2', 'value2');
 			element.setAttribute('key3', 'value3');
diff --git a/packages/happy-dom/test/tree-walker/NodeIterator.test.ts b/packages/happy-dom/test/tree-walker/NodeIterator.test.ts
index 9619dea56..26f571839 100644
--- a/packages/happy-dom/test/tree-walker/NodeIterator.test.ts
+++ b/packages/happy-dom/test/tree-walker/NodeIterator.test.ts
@@ -65,6 +65,47 @@ describe('NodeIterator', () => {
 			]);
 		});
 
+		it('Walks into each node in the DOM tree when elements are removed while iterating.', () => {
+			const nodeIterator = document.createNodeIterator(document.body);
+			const html: string[] = [];
+			let currentNode;
+
+			while ((currentNode = nodeIterator.nextNode())) {
+				if (currentNode.tagName === 'B') {
+					currentNode.remove();
+				}
+				html.push(NODE_TO_STRING(currentNode));
+			}
+
+			// We expect that the text inside the <b> tag is skipped as it is not connected to the DOM tree.
+
+			expect(html).toEqual([
+				'<body>\n\t\t\t<div class="class1 class2" id="id">\n\t\t\t\t<!-- Comment 1 !-->\n\t\t\t\t<b>Bold</b>\n\t\t\t\t<!-- Comment 2 !-->\n\t\t\t\t<span>Span</span>\n\t\t\t</div>\n\t\t\t<article class="class1 class2" id="id">\n\t\t\t\t<!-- Comment 1 !-->\n\t\t\t\t<b>Bold</b>\n\t\t\t\t<!-- Comment 2 !-->\n\t\t\t</article>\n\t\t\n\t</body>',
+				'\n\t\t\t',
+				'<div class="class1 class2" id="id">\n\t\t\t\t<!-- Comment 1 !-->\n\t\t\t\t<b>Bold</b>\n\t\t\t\t<!-- Comment 2 !-->\n\t\t\t\t<span>Span</span>\n\t\t\t</div>',
+				'\n\t\t\t\t',
+				'<!-- Comment 1 !-->',
+				'\n\t\t\t\t',
+				'<b>Bold</b>',
+				'\n\t\t\t\t',
+				'<!-- Comment 2 !-->',
+				'\n\t\t\t\t',
+				'<span>Span</span>',
+				'Span',
+				'\n\t\t\t',
+				'\n\t\t\t',
+				'<article class="class1 class2" id="id">\n\t\t\t\t<!-- Comment 1 !-->\n\t\t\t\t<b>Bold</b>\n\t\t\t\t<!-- Comment 2 !-->\n\t\t\t</article>',
+				'\n\t\t\t\t',
+				'<!-- Comment 1 !-->',
+				'\n\t\t\t\t',
+				'<b>Bold</b>',
+				'\n\t\t\t\t',
+				'<!-- Comment 2 !-->',
+				'\n\t\t\t',
+				'\n\t\t\n\t'
+			]);
+		});
+
 		it('Walks into each HTMLElement in the DOM tree when whatToShow is set to NodeFilter.SHOW_ELEMENT.', () => {
 			const nodeIterator = document.createNodeIterator(document.body, NodeFilter.SHOW_ELEMENT);
 			const html: string[] = [];
diff --git a/packages/happy-dom/test/tree-walker/TreeWalker.test.ts b/packages/happy-dom/test/tree-walker/TreeWalker.test.ts
index cdffc7828..4c6e3e998 100644
--- a/packages/happy-dom/test/tree-walker/TreeWalker.test.ts
+++ b/packages/happy-dom/test/tree-walker/TreeWalker.test.ts
@@ -65,6 +65,30 @@ describe('TreeWalker', () => {
 			]);
 		});
 
+		it('Walks into each node in the DOM tree while an element is removed.', () => {
+			const treeWalker = document.createTreeWalker(document.body);
+			const html: string[] = [];
+			let currentNode;
+
+			while ((currentNode = treeWalker.nextNode())) {
+				if (currentNode.tagName === 'B') {
+					currentNode.remove();
+				}
+
+				html.push(NODE_TO_STRING(currentNode));
+			}
+
+			expect(html).toEqual([
+				'\n\t\t\t',
+				'<div class="class1 class2" id="id">\n\t\t\t\t<!-- Comment 1 !-->\n\t\t\t\t<b>Bold</b>\n\t\t\t\t<!-- Comment 2 !-->\n\t\t\t\t<span>Span</span>\n\t\t\t</div>',
+				'\n\t\t\t\t',
+				'<!-- Comment 1 !-->',
+				'\n\t\t\t\t',
+				'<b>Bold</b>',
+				'Bold'
+			]);
+		});
+
 		it('Walks into each HTMLElement in the DOM tree when whatToShow is set to NodeFilter.SHOW_ELEMENT.', () => {
 			const treeWalker = document.createTreeWalker(document.body, NodeFilter.SHOW_ELEMENT);
 			const html: string[] = [];
diff --git a/packages/happy-dom/test/xml-parser/XMLParser.test.ts b/packages/happy-dom/test/xml-parser/XMLParser.test.ts
index e3dbadb55..6d53b9371 100644
--- a/packages/happy-dom/test/xml-parser/XMLParser.test.ts
+++ b/packages/happy-dom/test/xml-parser/XMLParser.test.ts
@@ -187,8 +187,8 @@ describe('XMLParser', () => {
 
 			expect((<HTMLElement>result.children[0].children[0]).textContent).toBe('');
 			expect((<HTMLElement>result.children[0].children[1]).textContent).toBe('');
-			expect((<HTMLElement>result.children[0].children[0]).innerHTML).toBe('<b></b>');
-			expect((<HTMLElement>result.children[0].children[1]).innerHTML).toBe('<b></b>');
+			expect((<HTMLElement>result.children[0].children[0]).innerHTML).toBe('<b/>');
+			expect((<HTMLElement>result.children[0].children[1]).innerHTML).toBe('<b/>');
 		});
 
 		it('Outputs error for incomplete end tag.', () => {