Free HTML Validator - Check HTML Errors Online
Validate HTML for errors, warnings, and accessibility issues in your browser.
Input
Validation Results
What Is an HTML Validator?
If you've ever spent an hour tracking down a layout bug only to find a missing closing </div>, you know why validation matters. An HTML validator checks your markup against the rules defined in the HTML Living Standard and flags common mistakes: unclosed tags, missing alt attributes on images (an WCAG accessibility requirement), deprecated elements like <font> and <center>, and missing DOCTYPE declarations. Catching these issues early prevents rendering bugs and accessibility problems across browsers.
Your markup is checked by JavaScript on this page, so the HTML you paste stays on your machine. The checks cover three things: structural errors such as unclosed tags, accessibility warnings like an img with no alt text, and elements the spec has deprecated.
How to Use This Tool
Paste or Upload HTML
Drop a page or a fragment of markup into the left-hand editor. Upload opens an .html file from disk, and Sample loads a snippet with a few deliberate mistakes in it.
Review Results
Findings appear on the right as soon as there is markup to check, with no button to press. Errors are prefixed with ✗ ERROR, warnings with ⚠ WARNING, and informational messages with ℹ INFO.
Fix Issues and Re-validate
Correct the issues in the left editor and the results update in real-time. To format your HTML for readability, try the HTML Formatter tool.
HTML Validation Example
Below is a snippet with common issues and the resulting validation output:
HTML with issues
Validation output
Why HTML Validation Matters
Invalid HTML can cause browsers to enter quirks mode, leading to inconsistent rendering across browsers. Missing alt attributes on images cause accessibility failures. Unclosed tags can break layout in unexpected ways.
For authoritative validation, use the W3C Markup Validation Service. This tool provides quick client-side checks for common issues.
Frequently Asked Questions
Does this replace the W3C Validator?
Not entirely. This tool provides instant client-side checks for the most common issues — no need to submit a URL or wait for a server response. For full spec-level validation, use the W3C Markup Validation Service. Many developers use both: this tool for quick checks during development, and the W3C service for final sign-off.
Is my HTML sent to a server?
No. Every check is done by script inside this tab, so the markup is never uploaded. If you want proof, open the Network tab in Chrome DevTools.
Which problems will it flag?
It catches unclosed tags (like <div> without a closing </div>), orphan closing tags, missing alt attributes on images, deprecated elements (<font>, <center>, <marquee>), and missing <!DOCTYPE html> declarations.
Can I validate a full HTML page or just snippets?
Both. Paste a full <html>...</html> document or just a fragment like a component template. The validator adapts to whatever you give it.
Related Tools
See the W3C Markup Validation Service for authoritative HTML validation. The HTML Living Standard is the reference specification.