HTML Validator & Formatter
Validate HTML structure, check for accessibility and SEO issues, and format your markup with proper indentation.
Structure
HTML documents should include DOCTYPE, html, head, title, and body tags.
Accessibility
Images should have alt attributes. Links should have descriptive text.
Best Practices
Use semantic tags (header, nav, main, footer) and close all tags properly.
About the HTML Validator
This HTML validator finds unclosed and mismatched tags, checks nesting, and flags accessibility problems such as missing alt attributes and unlabelled form inputs — the issues browsers hide by silently recovering.
An HTML validator is useful precisely because browsers are forgiving. A missing closing tag rarely produces a visible error; the browser guesses, and the guess is often not what you intended — which is why a layout breaks in one browser and not another.
Nesting rules are the substantive part. A block element inside a paragraph, a div inside a span, or a table cell outside a row are all invalid and get silently restructured during parsing. That restructuring is what makes the resulting DOM differ from the markup you wrote.
The accessibility checks catch the omissions with real consequences: an image with no alt attribute is invisible to a screen reader, and a form input with no associated label cannot be identified by one. Both are trivially fixable and easy to miss.
How to use the HTML Validator
- Paste your HTML. Enter the markup or page source.
- Read the tag findings. Check for unclosed, mismatched or wrongly nested tags.
- Review the accessibility notes. Look at missing alt text and unlabelled inputs.
- Fix and revalidate. Correct the markup and check again.
HTML Validator features
- Unclosed and mismatched tag detection
- Nesting rule checks
- Missing alt attribute warnings
- Form label association checks
- Attribute validation
- Runs entirely in your browser
Frequently asked questions
Why validate HTML if the page looks fine?
Browsers silently recover from errors by guessing. Different browsers guess differently, which is how a layout breaks in only one of them.
What nesting mistakes matter most?
A block element inside a paragraph, a div inside a span, or a table cell outside a row. All get silently restructured during parsing.
Which accessibility issues does it check?
Missing alt attributes on images and form inputs with no associated label — both trivial to fix and easy to miss.
Does it find unclosed tags?
Yes, along with tags closed in the wrong order, which is the usual cause of unexpected layout shifts.
Is my markup uploaded?
No. Validation happens entirely in your browser.