diff --git a/packages/happy-dom/src/html-parser/HTMLParser.ts b/packages/happy-dom/src/html-parser/HTMLParser.ts
index 6dc2ed98..c5f2ab46 100755
--- a/packages/happy-dom/src/html-parser/HTMLParser.ts
+++ b/packages/happy-dom/src/html-parser/HTMLParser.ts
@@ -402,7 +402,7 @@ export default class HTMLParser {
 					const name =
 						attributeMatch[1] || attributeMatch[3] || attributeMatch[6] || attributeMatch[9] || '';
 					const rawValue = attributeMatch[2] || attributeMatch[4] || attributeMatch[7] || '';
-					const value = rawValue ? XMLEncodeUtility.decodeAttributeValue(rawValue) : '';
+					const value = rawValue ? XMLEncodeUtility.decodeHTMLAttributeValue(rawValue) : '';
 					const attributes = this.nextElement[PropertySymbol.attributes];
 
 					if (this.nextElement[PropertySymbol.namespaceURI] === NamespaceURI.svg) {
diff --git a/packages/happy-dom/src/html-serializer/HTMLSerializer.ts b/packages/happy-dom/src/html-serializer/HTMLSerializer.ts
index 44b4f459..dede1814 100644
--- a/packages/happy-dom/src/html-serializer/HTMLSerializer.ts
+++ b/packages/happy-dom/src/html-serializer/HTMLSerializer.ts
@@ -147,11 +147,11 @@ export default class HTMLSerializer {
 
 		if (!namedItems.has('is') && element[PropertySymbol.isValue]) {
 			attributeString +=
-				' is="' + XMLEncodeUtility.encodeAttributeValue(element[PropertySymbol.isValue]) + '"';
+				' is="' + XMLEncodeUtility.encodeHTMLAttributeValue(element[PropertySymbol.isValue]) + '"';
 		}
 
 		for (const attributes of namedItems.values()) {
-			const escapedValue = XMLEncodeUtility.encodeAttributeValue(
+			const escapedValue = XMLEncodeUtility.encodeHTMLAttributeValue(
 				attributes[0][PropertySymbol.value]
 			);
 			attributeString += ' ' + attributes[0][PropertySymbol.name] + '="' + escapedValue + '"';
diff --git a/packages/happy-dom/src/query-selector/SelectorItem.ts b/packages/happy-dom/src/query-selector/SelectorItem.ts
index d27063ff..0bf5b4c5 100644
--- a/packages/happy-dom/src/query-selector/SelectorItem.ts
+++ b/packages/happy-dom/src/query-selector/SelectorItem.ts
@@ -7,6 +7,8 @@ import ISelectorAttribute from './ISelectorAttribute.js';
 import ISelectorMatch from './ISelectorMatch.js';
 import ISelectorPseudo from './ISelectorPseudo.js';
 
+const SPACE_REGEXP = /\s+/;
+
 /**
  * Selector item.
  */
@@ -417,7 +419,7 @@ export default class SelectorItem {
 			return null;
 		}
 
-		const classList = element.className.split(' ');
+		const classList = element.className.split(SPACE_REGEXP);
 		let priorityWeight = 0;
 
 		for (const className of this.classNames) {
diff --git a/packages/happy-dom/src/utilities/XMLEncodeUtility.ts b/packages/happy-dom/src/utilities/XMLEncodeUtility.ts
index 67ad9c86..8aef0b06 100644
--- a/packages/happy-dom/src/utilities/XMLEncodeUtility.ts
+++ b/packages/happy-dom/src/utilities/XMLEncodeUtility.ts
@@ -8,7 +8,7 @@ export default class XMLEncodeUtility {
 	 * @param value Value.
 	 * @returns Escaped value.
 	 */
-	public static encodeAttributeValue(value: string | null): string {
+	public static encodeXMLAttributeValue(value: string | null): string {
 		if (value === null) {
 			return '';
 		}
@@ -28,7 +28,7 @@ export default class XMLEncodeUtility {
 	 * @param value Value.
 	 * @returns Decoded value.
 	 */
-	public static decodeAttributeValue(value: string | null): string {
+	public static decodeXMLAttributeValue(value: string | null): string {
 		if (value === null) {
 			return '';
 		}
@@ -43,6 +43,33 @@ export default class XMLEncodeUtility {
 			.replace(/&/gu, '&');
 	}
 
+	/**
+	 * Encodes attribute value.
+	 *
+	 * @param value Value.
+	 * @returns Escaped value.
+	 */
+	public static encodeHTMLAttributeValue(value: string | null): string {
+		if (value === null) {
+			return '';
+		}
+		return value.replace(/&/gu, '&').replace(/"/gu, '"');
+	}
+
+	/**
+	 * Decodes attribute value.
+	 *
+	 * @param value Value.
+	 * @returns Decoded value.
+	 */
+	public static decodeHTMLAttributeValue(value: string | null): string {
+		if (value === null) {
+			return '';
+		}
+
+		return value.replace(/"/gu, '"').replace(/&/gu, '&');
+	}
+
 	/**
 	 * Encodes text content.
 	 *
diff --git a/packages/happy-dom/src/xml-parser/XMLParser.ts b/packages/happy-dom/src/xml-parser/XMLParser.ts
index ea051a38..39bba8d1 100755
--- a/packages/happy-dom/src/xml-parser/XMLParser.ts
+++ b/packages/happy-dom/src/xml-parser/XMLParser.ts
@@ -487,7 +487,7 @@ export default class XMLParser {
 
 					// In XML, new line characters should be replaced with a space.
 					const value = rawValue
-						? XMLEncodeUtility.decodeAttributeValue(rawValue.replace(NEW_LINE_REGEXP, ' '))
+						? XMLEncodeUtility.decodeXMLAttributeValue(rawValue.replace(NEW_LINE_REGEXP, ' '))
 						: '';
 					const attributes = this.nextElement[PropertySymbol.attributes];
 					const nameParts = name.split(':');
diff --git a/packages/happy-dom/src/xml-serializer/XMLSerializer.ts b/packages/happy-dom/src/xml-serializer/XMLSerializer.ts
index f568a5b9..75346e2e 100644
--- a/packages/happy-dom/src/xml-serializer/XMLSerializer.ts
+++ b/packages/happy-dom/src/xml-serializer/XMLSerializer.ts
@@ -229,7 +229,7 @@ export default class XMLSerializer {
 					attribute[PropertySymbol.localName] === elementPrefix &&
 					element[PropertySymbol.namespaceURI]
 				) {
-					namespaceString += ` xmlns:${elementPrefix}="${XMLEncodeUtility.encodeAttributeValue(
+					namespaceString += ` xmlns:${elementPrefix}="${XMLEncodeUtility.encodeXMLAttributeValue(
 						element[PropertySymbol.namespaceURI]
 					)}"`;
 					handledNamespaces.add(element[PropertySymbol.namespaceURI]);
@@ -238,20 +238,20 @@ export default class XMLSerializer {
 					attribute[PropertySymbol.name] === 'xmlns' &&
 					element[PropertySymbol.namespaceURI]
 				) {
-					namespaceString += ` xmlns="${XMLEncodeUtility.encodeAttributeValue(
+					namespaceString += ` xmlns="${XMLEncodeUtility.encodeXMLAttributeValue(
 						element[PropertySymbol.namespaceURI]
 					)}"`;
 					handledNamespaces.add(element[PropertySymbol.namespaceURI]);
 				} else {
 					namespaceString += ` ${
 						attribute[PropertySymbol.name]
-					}="${XMLEncodeUtility.encodeAttributeValue(attribute[PropertySymbol.value])}"`;
+					}="${XMLEncodeUtility.encodeXMLAttributeValue(attribute[PropertySymbol.value])}"`;
 					handledNamespaces.add(attribute[PropertySymbol.value]);
 				}
 			} else {
 				attributeString += ` ${
 					attribute[PropertySymbol.name]
-				}="${XMLEncodeUtility.encodeAttributeValue(attribute[PropertySymbol.value])}"`;
+				}="${XMLEncodeUtility.encodeXMLAttributeValue(attribute[PropertySymbol.value])}"`;
 			}
 		}
 
@@ -262,14 +262,14 @@ export default class XMLSerializer {
 			!handledNamespaces.has(element[PropertySymbol.namespaceURI])
 		) {
 			if (elementPrefix && !inheritedNamespacePrefixes.has(element[PropertySymbol.namespaceURI])) {
-				namespaceString += ` xmlns:${elementPrefix}="${XMLEncodeUtility.encodeAttributeValue(
+				namespaceString += ` xmlns:${elementPrefix}="${XMLEncodeUtility.encodeXMLAttributeValue(
 					element[PropertySymbol.namespaceURI]
 				)}"`;
 			} else if (
 				!elementPrefix &&
 				inheritedDefaultNamespace !== element[PropertySymbol.namespaceURI]
 			) {
-				namespaceString += ` xmlns="${XMLEncodeUtility.encodeAttributeValue(
+				namespaceString += ` xmlns="${XMLEncodeUtility.encodeXMLAttributeValue(
 					element[PropertySymbol.namespaceURI]
 				)}"`;
 			}
diff --git a/packages/happy-dom/test/html-parser/HTMLParser.test.ts b/packages/happy-dom/test/html-parser/HTMLParser.test.ts
index 3c9806d8..a260b197 100644
--- a/packages/happy-dom/test/html-parser/HTMLParser.test.ts
+++ b/packages/happy-dom/test/html-parser/HTMLParser.test.ts
@@ -2122,5 +2122,25 @@ describe('HTMLParser', () => {
 				'<html><head></head><body>Test</body></html>'
 			);
 		});
+
+		it('Handles line breaks in attributes for #1678', () => {
+			const result = new HTMLParser(window).parse(
+				`            <div>
+                <button class="btn btn-secondary comment_reply" data-id="{{id}}" type="button">{{message_gui_reply}}</button> <button class="btn btn-secondary comment_collapse
+                 visually-hidden" type="button">{{message_gui_replies}}</button>
+            </div>`
+			);
+
+			expect(new HTMLSerializer().serializeToString(result)).toBe(
+				`            <div>
+                <button class="btn btn-secondary comment_reply" data-id="{{id}}" type="button">{{message_gui_reply}}</button> <button class="btn btn-secondary comment_collapse
+                 visually-hidden" type="button">{{message_gui_replies}}</button>
+            </div>`
+			);
+
+			const element = result.querySelector('div > .comment_collapse');
+
+			expect(element).toBe(result.children[0].children[1]);
+		});
 	});
 });
diff --git a/packages/happy-dom/test/html-serializer/HTMLSerializer.test.ts b/packages/happy-dom/test/html-serializer/HTMLSerializer.test.ts
index 4efbe4c5..6ae234a9 100644
--- a/packages/happy-dom/test/html-serializer/HTMLSerializer.test.ts
+++ b/packages/happy-dom/test/html-serializer/HTMLSerializer.test.ts
@@ -254,7 +254,7 @@ describe('HTMLSerializer', () => {
 			div.setAttribute('attr3', '');
 
 			expect(serializer.serializeToString(div)).toBe(
-				'<div attr1="Hello ⁨John⁩" attr2="&lt;span&gt; test" attr3=""></div>'
+				'<div attr1="Hello ⁨John⁩" attr2="<span> test" attr3=""></div>'
 			);
 		});
 
diff --git a/packages/happy-dom/test/query-selector/QuerySelector.test.ts b/packages/happy-dom/test/query-selector/QuerySelector.test.ts
index a72324b5..7e4f9ea8 100644
--- a/packages/happy-dom/test/query-selector/QuerySelector.test.ts
+++ b/packages/happy-dom/test/query-selector/QuerySelector.test.ts
@@ -18,7 +18,7 @@ describe('QuerySelector', () => {
 		document = window.document;
 	});
 
-	describe('querySelectorAll', () => {
+	describe('querySelectorAll()', () => {
 		it('Throws an error for invalid selectors.', () => {
 			const container = document.createElement('div');
 			expect(() => container.querySelectorAll(<string>(<unknown>12))).toThrow(
@@ -1214,7 +1214,7 @@ describe('QuerySelector', () => {
 		});
 	});
 
-	describe('querySelector', () => {
+	describe('querySelector()', () => {
 		it('Throws an error for invalid selectors.', () => {
 			const container = document.createElement('div');
 			expect(() => container.querySelector(<string>(<unknown>12))).toThrow(
@@ -1628,6 +1628,16 @@ describe('QuerySelector', () => {
 			expect(document.querySelector(':focus')).toBe(div);
 			expect(document.querySelector(':focus-visible')).toBe(div);
 		});
+
+		it('Handles class names with line breaks', () => {
+			const div = document.createElement('div');
+			div.innerHTML = `
+                <div class="class1
+                class2"></div>
+            `;
+
+			expect(div.querySelector('.class1.class2')).toBe(div.children[0]);
+		});
 	});
 
 	describe('matches()', () => {