✅ HTML5 Validator

Detects syntax errors in HTML code. Reports forgotten closing tags, attribute errors, etc.

Enter HTML code

Usage and Application Examples

  • Validate by copying and pasting HTML files from a text editor
  • Automatic detection of closing tag correspondence errors
  • Detects the use of duplicate IDs
  • Warns against using deprecated tags
  • Check for required meta tags

What is HTML Validator?

An HTML5 Validator checks your HTML code for syntax errors, invalid attributes, missing closing tags, and standards compliance. Valid HTML ensures consistent rendering across browsers, improves accessibility for assistive technologies, and strengthens SEO performance. This free online validator instantly identifies and explains errors in your markup—from structural problems to deprecated elements—helping you write cleaner, more maintainable code that meets W3C standards.

How to Use

Paste your HTML code into the validator's text area or upload an HTML file. Click "Validate" and the tool scans your markup against HTML5 specifications. Results display a list of all errors, warnings, and notices, with line numbers and descriptions. Each issue explains what's wrong and how to fix it. For critical errors (unclosed tags, missing attributes), the validator highlights severity. You can expand each error for detailed guidance. Address issues starting with errors, then warnings, working through the list until your HTML passes validation with zero errors.

Use Cases

• Quality Assurance Before Launch: Run validation on all site pages before pushing to production to catch markup errors that could break layouts or confuse screen readers.
• Debugging Unexpected Behavior: When a page displays incorrectly despite correct CSS, validation often reveals hidden HTML problems causing rendering issues across browsers.
• Accessibility Compliance: Validation catches missing alt attributes, improperly nested elements, and semantic problems that hinder users relying on screen readers and keyboard navigation.
• Learning Best Practices: Beginning web developers use validation as an immediate feedback mechanism for writing standards-compliant HTML, reinforcing best practices.

Tips & Insights

Valid HTML doesn't guarantee perfect rendering (CSS issues exist separately), but invalid markup often causes cascading problems. Some errors are browser-forgiving (invalid nesting), while others break functionality. The validator differentiates between errors (don't use invalid syntax), warnings (outdated practices), and info messages (optimization suggestions). Validate frequently during development rather than once at the end; fixing markup early prevents compounding issues. Third-party widgets sometimes inject invalid HTML—validate your actual output, not just source code, to catch these problems.

Frequently Asked Questions

What does this tool check?

Checks for common errors in HTML code, such as support for closing tags, attribute syntax, duplicate IDs, use of deprecated tags, and presence of meta tags.

Detect all errors?

Major errors are detected, but for complete validation we recommend the use of specialized tools such as the official W3C validator or HTML Validator.

What is the difference between an error and a warning?

An Error is a problem that must be corrected, and a Warning is a problem that is recommended to be corrected.

Is data sent to the server?

No, all processing is completed within the browser; no HTML code is sent to the server.

Can I validate HTML from a live website URL?

This tool validates HTML code you paste directly into it. To validate a live URL, use the official W3C HTML Validator at validator.w3.org which has a direct URL input option. Alternatively, you can view page source from your browser, copy it, and paste it here.

What's the difference between errors and warnings in validation?

Errors are violations of HTML5 specification that can break functionality or accessibility. Warnings indicate non-standard practices that might cause issues in certain browsers or situations. Fixing all errors is essential; warnings should be reviewed based on your target audience and browser support requirements.

Does the validator check for accessibility issues (WCAG)?

This HTML validator checks for basic HTML5 spec compliance but doesn't perform full accessibility audits. For WCAG compliance checking, use specialized tools like WAVE, Axe, or Lighthouse. However, valid semantic HTML is a good foundation for accessibility.

Can I validate partial HTML code snippets or components?

Yes, you can paste HTML fragments like `<div>` blocks or component code. The validator will report any errors within that snippet. Note that some errors might appear due to missing parent elements—wrap your snippet in basic document structure for more accurate results.

What HTML standards does this validator follow?

This tool validates against HTML5 specifications, the current web standard. It checks for properly closed tags, valid attributes, required elements, and semantic correctness. Older HTML4 or XHTML documents may report errors if they don't conform to HTML5 rules.

How do I fix the most common HTML errors reported?

Common errors include unclosed tags (add missing `</tag>`), invalid nesting (fix element hierarchy), missing required attributes, and typos in tag names. Check the validator's detailed error messages for exact line numbers and suggestions. Using an IDE with HTML linting features can prevent these errors during development.