fix: [#1949] Implement implicit closing of <p> elements per HTML spec

packages/happy-dom/src/config/HTMLElementConfig.ts

@@ -331,7 +331,40 @@ export default <
 	},
 	p: {
 		className: 'HTMLParagraphElement',
-		contentModel: HTMLElementConfigContentModelEnum.anyDescendants
+		contentModel: HTMLElementConfigContentModelEnum.noForbiddenFirstLevelDescendants,
+		forbiddenDescendants: [
+			'address',
+			'article',
+			'aside',
+			'blockquote',
+			'details',
+			'dialog',
+			'div',
+			'dl',
+			'fieldset',
+			'figcaption',
+			'figure',
+			'footer',
+			'form',
+			'h1',
+			'h2',
+			'h3',
+			'h4',
+			'h5',
+			'h6',
+			'header',
+			'hgroup',
+			'hr',
+			'main',
+			'menu',
+			'nav',
+			'ol',
+			'p',
+			'pre',
+			'section',
+			'table',
+			'ul'
+		]
 	},
 	param: {
 		className: 'HTMLParamElement',

packages/happy-dom/test/html-parser/HTMLParser.malformedHTML.test.ts

@@ -0,0 +1,106 @@
+import Window from '../../src/window/Window.js';
+import Document from '../../src/nodes/document/Document.js';
+import HTMLParser from '../../src/html-parser/HTMLParser.js';
+import HTMLSerializer from '../../src/html-serializer/HTMLSerializer.js';
+import { beforeEach, describe, it, expect } from 'vitest';
+
+/**
+ * Test cases for GitHub issue #1949:
+ * DOMParser does not correctly handle malformed HTML
+ *
+ * Per the HTML spec, a <p> element is implicitly closed when certain
+ * block-level elements are encountered.
+ *
+ * @see https://github.com/capricorn86/happy-dom/issues/1949
+ * @see https://html.spec.whatwg.org/C/grouping-content.html
+ */
+describe('HTMLParser - Malformed HTML handling (Issue #1949)', () => {
+	let window: Window;
+
+	beforeEach(() => {
+		window = new Window();
+	});
+
+	describe('Paragraph element implicit closing', () => {
+		it('Should handle the original issue: stray </div> with nested <p>', () => {
+			const doc = new window.DOMParser().parseFromString(
+				'<p>testing with </div><p>new line</p>',
+				'text/html'
+			);
+			expect(doc.body.innerHTML).toBe('<p>testing with </p><p>new line</p>');
+		});
+
+		it('Should close <p> when another <p> is encountered', () => {
+			const doc = new window.DOMParser().parseFromString(
+				'<p>first<p>second<p>third</p>',
+				'text/html'
+			);
+			expect(doc.body.innerHTML).toBe('<p>first</p><p>second</p><p>third</p>');
+		});
+
+		it('Should close <p> when block-level elements are encountered', () => {
+			// Test representative block elements: div, heading, list, table, sectioning
+			const testCases = [
+				{ input: '<p>text<div>block</div>', expected: '<p>text</p><div>block</div>' },
+				{ input: '<p>text<h1>heading</h1>', expected: '<p>text</p><h1>heading</h1>' },
+				{ input: '<p>text<ul><li>item</li></ul>', expected: '<p>text</p><ul><li>item</li></ul>' },
+				{
+					input: '<p>text<table><tr><td>cell</td></tr></table>',
+					expected: '<p>text</p><table><tbody><tr><td>cell</td></tr></tbody></table>'
+				},
+				{
+					input: '<p>text<section>content</section>',
+					expected: '<p>text</p><section>content</section>'
+				},
+				{ input: '<p>text<hr>', expected: '<p>text</p><hr>' },
+				{
+					input: '<p>text<blockquote>quote</blockquote>',
+					expected: '<p>text</p><blockquote>quote</blockquote>'
+				}
+			];
+
+			for (const { input, expected } of testCases) {
+				const doc = new window.DOMParser().parseFromString(input, 'text/html');
+				expect(doc.body.innerHTML).toBe(expected);
+			}
+		});
+
+		it('Should NOT close <p> when inline elements are encountered', () => {
+			const testCases = [
+				{ input: '<p>text<span>inline</span></p>', expected: '<p>text<span>inline</span></p>' },
+				{ input: '<p>text<a href="#">link</a></p>', expected: '<p>text<a href="#">link</a></p>' },
+				{ input: '<p>text<strong>bold</strong></p>', expected: '<p>text<strong>bold</strong></p>' }
+			];
+
+			for (const { input, expected } of testCases) {
+				const doc = new window.DOMParser().parseFromString(input, 'text/html');
+				expect(doc.body.innerHTML).toBe(expected);
+			}
+		});
+
+		it('Should handle nested structures correctly', () => {
+			const doc = new window.DOMParser().parseFromString(
+				'<div><p>first<p>second</p></div>',
+				'text/html'
+			);
+			expect(doc.body.innerHTML).toBe('<div><p>first</p><p>second</p></div>');
+		});
+	});
+
+	describe('Stray end tags', () => {
+		it('Should ignore stray end tags with no matching open tag', () => {
+			const doc = new window.DOMParser().parseFromString(
+				'<div>content</span></section></div>',
+				'text/html'
+			);
+			expect(doc.body.innerHTML).toBe('<div>content</div>');
+		});
+	});
+
+	describe('Fragment parsing', () => {
+		it('Should handle malformed HTML in fragments', () => {
+			const result = new HTMLParser(window).parse('<p>first<p>second</p>');
+			expect(new HTMLSerializer().serializeToString(result)).toBe('<p>first</p><p>second</p>');
+		});
+	});
+});