The Security Checklist - The Practical Security Checklist for Web Developers
W3C Markup Validation Service - Validates HTML markup of web pages, one at a time.
SafeBase - SafeBase is the interactive security portal to share your security posture, automate access to sensitive compliance information, and subscribe to vendor security program updates.
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.
WordPress Security Scanner - Check if your WordPress site has known vulnerabilities
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.