HTML Validator — Check Your HTML Code for Errors
Use an HTML validator to find and fix errors in your code. Learn about the best free validation tools and common HTML mistakes.
An HTML validator checks your markup against web standards and identifies errors like unclosed tags, missing attributes, and incorrect nesting. Clean HTML loads faster, displays correctly across browsers, and ranks better in search engines.
Why Validate HTML?
- Cross-browser compatibility — Valid HTML renders consistently in Chrome, Firefox, Safari, and Edge
- Accessibility — Validators catch missing alt attributes, improper heading hierarchy, and other accessibility issues
- SEO — Search engines prefer well-structured HTML
- Debugging — Find errors that cause layout problems or broken functionality
Best Free HTML Validators
W3C Markup Validation Service
The official validator from the World Wide Web Consortium. Paste your HTML, upload a file, or enter a URL.
- URL: validator.w3.org
- Checks against current HTML standards
- Provides detailed error descriptions with line numbers
HTMLHint
A static analysis tool for HTML that checks for common mistakes and best practices. Available as a VS Code extension or command-line tool.
Browser DevTools
Chrome and Firefox developer tools highlight HTML errors in the Elements panel. Look for red highlighting on malformed elements.
VS Code Extensions
- HTMLHint — Real-time error highlighting
- W3C Web Validator — Run W3C checks from your editor
Common HTML Errors
- Unclosed tags —
<p>Textwithout</p> - Missing alt attributes —
<img src="photo.jpg">withoutalt="..." - Improper nesting —
<b><i>text</b></i>instead of<b><i>text</i></b> - Duplicate IDs — Using the same
idon multiple elements - Missing doctype — Every HTML page should start with
<!DOCTYPE html>
Testing Your HTML
After fixing validation errors, preview your page in a browser. You can also upload your HTML file to Linkyhost's HTML viewer to see how it renders online and share it with others for review.
Tips
- Validate early and often during development
- Fix errors from top to bottom — one error can cascade into many
- Use an editor with real-time validation to catch mistakes as you type
- Valid HTML is not the same as good HTML — validation checks syntax, not design quality