Free HTML Validator - Check HTML Code Online (W3C Standards)
Validate HTML code against W3C standards. Find syntax errors, missing tags, and accessibility issues. Free online HTML validator for cross-browser compatibility and SEO optimization.
HTML Input
Validation Results
Enter HTML code and click "Validate HTML" to see results
Sample HTML Code
About Free HTML Validator - Check HTML Code Online (W3C Standards)
HTML validation tool that checks your markup against W3C HTML5 standards. The validator identifies syntax errors, deprecated tags, missing attributes, unclosed elements, and accessibility issues that could affect your site's layout or SEO performance. Get detailed error reports with line-by-line references to help you fix issues. Use it to validate websites, landing pages, email templates, and any HTML document. All validation happens in your browser, so your code stays on your device and doesn't go to our servers.
Problems We Solve
- Browser rendering inconsistencies that cause layout to display differently across Chrome, Firefox, Safari, and Edge
- Accessibility issues that prevent WCAG 2.1 compliance and make sites unusable for people with disabilities
- HTML structure problems that prevent search engines from properly crawling and indexing your content
- Time spent debugging display bugs caused by unclosed tags or invalid element nesting
- Code reviews that fail due to non-compliant markup and missing quality standards
- Mobile responsiveness issues from invalid HTML that renders differently on various devices
- Slow page load times from error-filled code that browsers must correct before rendering
- Screen reader incompatibility that makes content inaccessible to visually impaired users
Why use a Free HTML Validator - Check HTML Code Online (W3C Standards)?
Valid HTML ensures consistent rendering across all browsers and devices, which eliminates cross-browser bugs. Search engines use HTML structure as a signal when ranking pages - well-structured code helps crawlers properly index your content and can improve your rankings. For accessibility, valid HTML is necessary for screen readers and assistive technologies to work correctly, which helps you comply with WCAG standards and avoid legal risks. Validated code loads faster because browsers don't need to spend time correcting errors. Valid HTML also demonstrates code quality to clients and passes code reviews. Validating takes less than a minute but can save hours of debugging by catching errors before deployment.
Who is it for?
Frontend developers building websites and web applications who need to ensure code quality before deployment. Web designers converting designs into HTML that works across all browsers. QA testers validating markup quality and catching errors during testing. Freelancers and agencies who deliver client projects and need standards-compliant code. Content creators and marketers building landing pages who need SEO-friendly and accessible HTML. Accessibility specialists ensuring WCAG compliance and screen reader compatibility. Students and bootcamp graduates learning web development. Technical writers creating HTML documentation and email developers building responsive email templates.
How to use the tool
Paste your HTML code into the text area or upload an HTML file
Click 'Validate' to analyze your code
Review the error report which shows errors organized by severity with line numbers
Fix critical errors first (these break functionality or violate standards)
Fix warnings to improve code quality and follow best practices
Review accessibility notices to ensure your HTML works with assistive technologies
Run validation again after making corrections to confirm issues are fixed
Key Features
W3C Standards Validation
Validates against HTML5, XHTML 1.0/1.1, and HTML 4.01 standards. Detects syntax violations, deprecated elements, and invalid attributes.
Detailed Error Reports with Line Numbers
Shows exact error locations. Classifies issues by severity (errors vs. warnings) and explains each problem.
Accessibility Compliance Checking
Detects accessibility issues including missing alt attributes, improper heading structure, missing ARIA labels, and WCAG 2.1 violations.
Browser Compatibility Analysis
Identifies HTML elements and attributes that may cause rendering problems in specific browsers.
Multiple Input Methods
Validate by pasting code, uploading HTML files, or entering a URL to check live websites.
SEO Structure Analysis
Checks for SEO-important HTML elements including heading hierarchy, meta tags, semantic HTML5 elements, and structured data markup.
Common Use Cases
Pre-Deployment Quality Assurance
Scenario: Before deploying code to production, validate it to catch errors that could break layouts, cause browser incompatibilities, or hurt SEO rankings.
✓ Catch and fix errors before they affect users.
Client Project Verification
Scenario: Freelancers and agencies can validate client projects before delivery to ensure the code is standards-compliant and passes technical reviews.
✓ Deliver higher quality work and reduce revision requests.
Legacy Code Updates
Scenario: When updating old websites, identify deprecated HTML elements, outdated attributes, and non-standard markup that needs updating.
✓ Systematically upgrade old codebases to current web standards.
Accessibility Compliance
Scenario: Validate HTML to meet ADA, Section 508, and WCAG requirements and reduce legal liability.
✓ Ensure your site is usable by all visitors and meets legal requirements.
SEO Optimization
Scenario: Validate landing pages to ensure proper heading structure, semantic HTML, and error-free markup that helps search engine rankings.
✓ Improve search engine rankings with clean, well-structured HTML.
Learning Web Development
Scenario: Students and junior developers can validate their code to learn proper HTML syntax and understand error messages.
✓ Learn proper HTML by getting feedback on your code.
Why Choose This Tool
- No registration or account required to use
- No usage limits or paywalls
- Works on mobile devices, tablets, and desktop computers
- Supports multiple HTML standards and doctypes
- Includes accessibility checking in addition to syntax validation
Frequently Asked Questions
What is HTML validation and why is it important?
HTML validation checks your code against official W3C (World Wide Web Consortium) standards to ensure it's properly structured and error-free. It's important because valid HTML prevents browser compatibility issues, helps search engines properly crawl and index your content, ensures accessibility for users with disabilities, reduces debugging time, and demonstrates professional code quality. Invalid HTML can cause rendering problems, broken layouts, and accessibility violations.
How do I validate my HTML code?
Paste your HTML code into the text area, upload an HTML file, or enter a URL to check a live website. Click the 'Validate' button to receive a detailed report showing any syntax errors, missing tags, deprecated elements, or standards violations. Each error includes the line number, error type, and an explanation of what's wrong and how to fix it.
Is HTML validation required for SEO?
Valid HTML isn't a direct ranking factor, but it affects SEO indirectly. Clean HTML improves page load speed (which is a ranking factor), helps search engines properly crawl and parse your content, prevents rendering issues that hurt user experience, and enables proper indexing of structured data. Sites with major HTML errors often perform worse in search results.
What HTML standards does this validator check?
The validator checks against HTML5 (the current standard), XHTML 1.0/1.1, HTML 4.01, and earlier versions as defined by W3C. It identifies syntax errors, deprecated tags, missing required attributes, improper element nesting, and accessibility issues. The validator detects your DOCTYPE declaration and validates accordingly.
Can I use this validator for email HTML templates?
Yes, the validator works for HTML email templates. However, email clients (especially Outlook) have different HTML requirements than web browsers and often support more limited HTML. The validator will flag standard HTML errors and accessibility issues, but you should also test rendered emails in actual email clients.
Does this validator automatically fix HTML errors?
No, the validator identifies and explains errors but doesn't auto-fix them. Automatically fixing errors could introduce unintended changes and wouldn't help you learn proper HTML. Instead, we provide error messages with line numbers, severity levels, and instructions on how to manually correct each issue.
Is this HTML validator free?
Yes, completely free with no hidden costs, no registration required, and unlimited validations. You can validate as many HTML documents as you need.
What's the difference between HTML errors and warnings?
Errors are critical issues that violate HTML standards and will likely cause problems - these include syntax errors, unclosed tags, invalid attributes, or improper element nesting. Browsers may still render pages with errors, but results are unpredictable. Warnings are less severe issues that don't violate standards but aren't best practices - like missing optional attributes or deprecated elements that still work. Fix all errors; address warnings based on your project requirements.
Does your validator check for accessibility (WCAG) compliance?
Yes, the validator includes checks for common WCAG 2.1 issues like missing alt text on images, improper heading hierarchy, missing form labels, and missing ARIA attributes. However, for comprehensive accessibility audits, use this validator alongside dedicated accessibility testing tools, as WCAG compliance involves more than just HTML validation.
Can I validate HTML generated by JavaScript frameworks?
Yes, but you need to validate the rendered HTML output, not the source code. For React, Vue, Angular, or other frameworks, use your browser's developer tools to view the generated HTML in the DOM, then copy and paste that into our validator. If you're validating a live URL, the validator will check the rendered HTML that browsers see.
Why does my site work fine but the validator shows errors?
Browsers are forgiving and use error correction to display pages even with invalid HTML. Just because a page looks fine doesn't mean the HTML is valid - browsers guess how to handle errors, and different browsers may guess differently. Invalid HTML can cause inconsistent rendering across browsers, slower page loads (due to error correction), SEO issues, accessibility problems, and unpredictable behavior. Fixing errors ensures consistent performance across all browsers and devices.
Technical Specifications
Supported Formats
- ✓HTML5, XHTML 1.0/1.1, HTML 4.01, and earlier versions
- ✓UTF-8, UTF-16, ISO-8859-1, and common character encodings
- ✓Inline CSS and JavaScript validation context
- ✓SVG and MathML embedded in HTML5
- ✓Standard DOCTYPE declarations
- ✓ARIA attributes and roles for accessibility
- ✓Responsive meta tags and viewport declarations
Pro Tips
- Include a DOCTYPE declaration at the start of your HTML document to specify which standard you're using
- Fix errors from top to bottom - early errors often cause cascading issues that disappear once the root cause is fixed
- Pay attention to closing tags - unclosed divs and spans are common HTML errors
- Use semantic HTML5 elements (header, nav, main, article, aside, footer) instead of generic divs for better SEO and accessibility
- Validate during development - catching errors early is easier than debugging later
- Check both desktop and mobile versions of responsive sites as they may have different HTML
- Run validation before deploying to production as part of your quality assurance process
- Keep a validation report for client deliverables to show code quality
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