Dr. Link Check - Broken link checker with malware and parked domain detection
Total Validator - Total Validator can check your website to help ensure that it is accessible, uses valid HTML and CSS, has no broken links, and is free from spelling mistakes.
Check a Site - Scan websites for over 700 quality problems
W3C Markup Validation Service - Validates HTML markup of web pages, one at a time.
Karate - Web-services testing made simple 🥋
Rocket Validator - Site-wide Accessibility and HTML validation made easy. With a single click, Rocket Validator will check thousands of web pages on your site for Accessibility and HTML conformance, producing a detailed, actionable report on seconds.