Meta Tags - Meta Tags is a tool to debug and generate meta tag code for any website.
W3C Markup Validation Service - Validates HTML markup of web pages, one at a time.
Devhints - TL;DR for developer documentation
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.
Meta Tag Generator - Generate HTML code optimal for SEO, social media, & mobile.
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.