Input HTML

Validation Results

What is HTML Validator?

HTML Validator checks your HTML markup for common errors like unclosed tags, missing required attributes, duplicate IDs, and improper nesting. It follows the rules defined in the WHATWG HTML Living Standard to help you write cleaner, more reliable web pages.

Whether you're building a new page, debugging a rendering issue, or preparing HTML for production, this tool catches mistakes that browsers silently fix but can still cause layout bugs or accessibility problems. For a deeper understanding of HTML standards, the web.dev Learn HTML guide is an excellent resource.

Everything runs in your browser. Your HTML never leaves your computer. Fast, free, and private.

How to Use the HTML Validator

Follow these steps to validate your HTML. Each step uses the actual buttons and panels on this page.

Paste, Upload, or Load Sample

Paste your HTML into the left Input HTML panel, click Upload to load a .html file, or click Sample to load example HTML with intentional errors.

<!DOCTYPE html>
<html>
<head><title>Test</title></head>
<body>
  <h1>Hello World</h1>
  <p>This is a test page.</p>
</body>
</html>

The validator checks your HTML automatically as you type.

Review Validation Results

The right Validation Results panel shows whether your HTML is valid or lists specific errors with descriptions. Green means valid, red means issues were found.

Fix and Revalidate

Use the error messages to fix issues in your HTML. The validator updates in real time as you edit. Click Copy to copy the validation results to share with your team.

When You'd Actually Use This

Debugging Rendering Issues

Page looks broken in one browser but fine in another? Unclosed tags, mismatched elements, and invalid nesting are common culprits. The validator catches these so you can fix the root cause instead of hacking around symptoms.

Ensuring Accessibility

Missing alt attributes on images, empty links, and improper heading hierarchy make your site harder to use with screen readers. This validator flags accessibility-related HTML issues so you can fix them early.

Checking Email Templates

Email HTML is notoriously fragile. Validate your email templates before sending to catch unclosed tags and structural issues that email clients handle inconsistently.

Validating CMS Output

Content management systems sometimes generate messy HTML. Run the output through this validator to catch duplicate IDs, broken nesting, and other issues before publishing.

Common Questions

What errors does this validator check for?

It checks for unclosed tags, missing required attributes (like alt on images), duplicate IDs, improperly nested elements, missing DOCTYPE declaration, missing document structure (html/head/body), and unclosed quotes in attributes.

Is this the same as the W3C Validator?

Not exactly. The W3C Markup Validation Service validates against the full HTML specification. This tool focuses on the most common and impactful errors that developers encounter daily. For full spec compliance, use both tools together.

Is my HTML data stored anywhere?

No. Everything runs entirely in your browser using JavaScript. Your HTML never leaves your computer.

Does it check CSS and JavaScript too?

No, this tool validates HTML markup only. For CSS validation, try our CSS Validator. For JavaScript, use our JS Validator.

Can I validate large HTML files?

Yes. Since validation happens in your browser, performance depends on your device. Files up to several megabytes validate quickly on modern hardware.

Does it support HTML5?

Yes. The validator understands HTML5 elements like <article>, <section>, <nav>, <header>, <footer>, and other semantic tags defined in the HTML Living Standard.

Related HTML & Markup Tools

This validator checks HTML for errors. Here are related tools for other HTML tasks: