The W3C Markup Validator
Find and fix HTML errors with the W3C Markup Validation Service.
What the Validator Checks
The W3C Markup Validator (validator.w3.org) parses HTML against the official specification and reports every spec violation: unclosed tags, invalid nesting (a <p> inside another <p>), missing required attributes (img without alt), unrecognized elements, and many others.
Why Validation Matters
Invalid HTML may render in any browser today, but each browser is free to recover differently — what looks fine in Chrome may break in Firefox. Valid HTML produces predictable, consistent rendering and avoids surprising behavior in screen readers, search crawlers, and email clients.
All lessons in this course
- The W3C Markup Validator
- Lighthouse HTML Audit
- Pa11y and axe for Accessibility Testing
- HTML Snapshot Testing