Screaming Frog SEO Spider - The Screaming Frog SEO Spider is a small desktop program (PC or Mac) which crawls websites’ links...
W3C Markup Validation Service - Validates HTML markup of web pages, one at a time.
SEMRush - All-in-one Marketing Toolkit for digital marketing professionals.
HTML Validator - HTML Validator is a Mozilla extension that adds HTML validation inside Firefox and Mozilla.
Pulno - SEO Audit and Website Analysis. Pull up your SEO.
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.