How to Validate HTML Markup
Invalid HTML can break layout, accessibility, and SEO. Unclosed tags, wrong nesting, or invalid attributes cause browsers to guess—and crawlers may misinterpret your page. Our HTML Validator highlights errors and suggests fixes.
Common causes
- Unclosed or mismatched tags (e.g. <div> without </div>).
- Block elements inside inline elements.
- Missing required attributes (e.g. alt on images).
- Invalid or deprecated attributes.
How to fix
- Paste HTML or enter a URL in the validator.
- Fix errors in order (one error can cause many others).
- Ensure all tags are properly closed and nested.
- Add alt to images and use semantic tags where possible.
Use our tool
Open HTML Validator