Nibbler - Free tool for testing how good your website is, and what you can do to improve it. Check accessibility, SEO, social media, compliance and more.
HTML Validator - HTML Validator is a Mozilla extension that adds HTML validation inside Firefox and Mozilla.
Pennywise - Cross-platform application to open any website or media in a floating window
W3C Markup Validation Service - Validates HTML markup of web pages, one at a time.
Zercurity - Monitor your infrastructure for policy non-compliance, live
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.