Check HTML for Accessibility Errors
HTML validation catches a meaningful subset of accessibility errors at the specification level: missing alt attributes on images, invalid ARIA role usage, missing labels on form inputs, and incorrect role-attribute combinations that the WAI-ARIA spec defines as errors.
HTML Validator & Linter
Our free HTML validator helps you check HTML5 code for syntax errors online. This html validator no download tool catches missing tags, invalid attributes, duplicate IDs, unclosed elements, and W3C HTML5 spec violations. All validation runs locally in your browser with WCAG and ARIA awareness, so your code stays private.
Check HTML5 code for errors, accessibility, and SEO issues instantly
- Checks
- Syntax & SEO
- Standards
- W3C HTML5
- Mode
- In-browser
- Price
- Free
W3C Standards
Validates against W3C HTML5 standards for complete compliance.
Accessibility
Checks for WCAG compliance and accessibility issues.
100% Private
Everything runs locally. Your HTML never leaves your device.
Validate HTML online
Paste your HTML code, click Validate, and review the results for errors, warnings, and suggestions.
Demo fetch uses a CORS-friendly approach only if the target allows it.
Privacy-first
This page processes content locally in your browser (no upload).
What is HTML Validation?
HTML validation is the process of checking HTML code against official W3C HTML5 standards to ensure it's syntactically correct, structurally sound, and follows web development best practices. Our free HTML validator tool analyzes your code to detect errors, warnings, and potential issues that could affect functionality, accessibility, SEO, or performance. This html validator online tool helps you validate HTML5 code online instantly.
When you write HTML code, it's easy to introduce mistakes like missing closing tags, invalid attributes, or structural errors. While modern browsers are forgiving and often render invalid HTML, these errors can cause unexpected behavior, accessibility problems, SEO issues, and compatibility problems across different browsers and devices. Our html syntax checker online helps you catch these issues before they become problems. Validate HTML before deployment to ensure your code is production-ready. For more HTML tools, check out our HTML Tools collection.
| Feature | Unvalidated HTML | Validated HTML |
|---|---|---|
| Browser Compatibility | Inconsistent rendering across browsers, layout breaks, broken functionality | Consistent rendering across all browsers and devices |
| Accessibility | Screen readers break, missing alt text, improper headings, WCAG violations | WCAG compliant, accessible to all users, proper semantic structure |
| SEO Impact | Poor crawlability, missing meta tags, improper heading hierarchy, lower rankings | Better crawlability, proper meta tags, semantic structure, improved rankings |
| Performance | Slower parsing, extra browser processing, poor Core Web Vitals | Faster parsing, optimized rendering, better Core Web Vitals scores |
| Security | Missing rel="noopener", potential XSS vulnerabilities, insecure practices | Security best practices, proper link attributes, reduced vulnerabilities |
| Maintainability | Hard to debug, unclear structure, difficult to maintain | Clear structure, easier debugging, better maintainability |
✗Invalid HTML
<!DOCTYPE html>
<html>
<head>
<title>Example</title>
</head>
<body>
<h1>Welcome</h1>
<p>This paragraph is not closed
<img src="image.jpg">
<a href="link.html">Click here
</body>
</html>Missing closing tags, missing alt text, unclosed elements
✓Valid HTML
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Example</title>
</head>
<body>
<h1>Welcome</h1>
<p>This paragraph is closed.</p>
<img src="image.jpg" alt="Description">
<a href="link.html">Click here</a>
</body>
</html>All tags closed, proper structure, accessibility attributes
What Does HTML Validation Check?
- •Syntax Errors: Missing closing tags, mismatched tags, invalid characters, and malformed HTML structure
- •Required Elements: DOCTYPE declaration, html tag, head tag, title tag, body tag, and other essential HTML5 elements
- •Accessibility Issues: Missing alt text on images, missing lang attribute, improper heading hierarchy, missing ARIA labels, and other WCAG compliance issues
- •SEO Problems: Missing meta description, missing Open Graph tags, improper heading structure, missing title tag, and other SEO best practices
- •Performance Warnings: Missing lazy loading on images, oversized images, missing optimization attributes, and other performance issues
- •Security Issues: External links missing rel="noopener", insecure resources, and other security best practices
According to W3C WCAG guidelines, valid HTML is the foundation of accessible web content. Invalid HTML can break screen readers, cause layout issues, and create barriers for users with disabilities. Our HTML validator checks for both syntax errors and accessibility compliance.
Modern web development workflows should include HTML validation as a standard step. Whether you're building a new website, maintaining existing code, or learning HTML, using an HTML validator helps ensure your code is correct, accessible, and optimized for search engines and performance. For more information on HTML standards, see the MDN HTML documentation and Google's HTML learning guide.
HTML Validation Impact
Real data showing the importance of validating HTML code
Validation Statistics
According to W3C research, over 85% of websites contain HTML validation errors. These errors can cause accessibility problems, SEO issues, browser compatibility problems, and unexpected rendering. Regular HTML validation helps catch and fix these issues before they impact users or search engine rankings.
Why Validate HTML?
Validating HTML code is essential for building reliable, accessible, and search-engine-friendly websites. Our free online HTML validator helps you check HTML code online for errors and issues. Here's why you should make HTML validation part of your development workflow:
Ensure Browser Compatibility
Invalid HTML can render differently across browsers. Chrome, Firefox, Safari, and Edge may handle errors inconsistently, leading to layout breaks, missing content, or broken functionality. Valid HTML ensures consistent rendering across all browsers and devices, reducing cross-browser testing time and user complaints.
Improve Accessibility
Invalid HTML breaks screen readers and assistive technologies. Missing alt text, improper heading hierarchy, and missing ARIA labels prevent users with disabilities from accessing your content. Valid HTML with proper semantic structure is the foundation of WCAG 2.1 compliance. This is not just best practice—it's often a legal requirement.
Boost SEO Rankings
Search engines like Google prefer valid, well-structured HTML. Missing meta tags, improper heading hierarchy, and invalid structure can hurt your search rankings. Validate HTML for SEO to ensure proper semantic structure. Valid HTML helps search engines understand and index your content better, potentially improving your rankings and organic traffic. Use our html validator for developers to catch SEO issues early, then run the HTML SEO Analyzer for a deeper audit of meta tags and on-page SEO.
Catch Errors Early
HTML validation catches errors before they cause problems in production. Missing closing tags, invalid attributes, and structural errors can lead to broken layouts, JavaScript failures, and user experience issues. Validating during development saves debugging time and prevents costly fixes after deployment.
Improve Performance
Invalid HTML can cause browsers to spend extra time parsing and fixing errors, slowing down page rendering. Valid HTML renders faster, improving Core Web Vitals metrics like First Contentful Paint (FCP) and Largest Contentful Paint (LCP). Faster pages provide better user experience and can improve search rankings.
Enhance Security
Valid HTML helps prevent security vulnerabilities. Missing rel="noopener" on external links can expose your site to tabnabbing attacks. Invalid HTML can also make your site more vulnerable to XSS attacks. Validating HTML helps ensure you're following security best practices and protecting your users.
How It Works
Our html validator browser tool uses client-side parsing and rule checking to validate HTML syntax online. This html validation tool free solution works instantly in your browser. Here's how the validation process works:
Parse HTML Structure
The validator parses your HTML code to identify all tags, attributes, and structure. It builds a tree representation of your document and checks for proper nesting and hierarchy.
Check Syntax Errors
The validator checks for missing closing tags, mismatched tags, invalid attributes, missing required elements (DOCTYPE, html, head, body, title), and other syntax errors that break HTML validity.
Validate Accessibility
The validator checks for accessibility issues including missing alt text on images, missing lang attribute, improper heading hierarchy (h1 should be first, no skipped levels), missing ARIA labels, and other WCAG compliance issues.
Check SEO & Performance
The validator checks for SEO issues (missing meta description, missing Open Graph tags, improper heading structure) and performance warnings (missing lazy loading, security issues with external links). It generates a comprehensive report with errors, warnings, and suggestions.
Best Practices for HTML Validation
Follow these best practices to ensure your HTML code is valid, accessible, and optimized. Our html validator online instant tool helps you check html errors online quickly. Use this html code validator online regularly to maintain code quality:
Always Include DOCTYPE
Every HTML document should start with <!DOCTYPE html>. This tells browsers which HTML version to use and ensures proper rendering. Without it, browsers may enter quirks mode, causing inconsistent rendering.
✅ DO: <!DOCTYPE html>
❌ DON'T: Skip DOCTYPE declaration
Close All Tags Properly
Every opening tag must have a corresponding closing tag (except self-closing tags like <img>, <br>). Mismatched or unclosed tags can break layout and functionality.
Test regularly: Validate HTML after major changes, before deployment, and as part of your build process
Use Semantic HTML
Use semantic HTML5 elements like <header>, <nav>, <main>, <section>, <article>, and <footer>. These improve accessibility, SEO, and code maintainability.
Semantic benefits: Better accessibility • Improved SEO • Easier maintenance • Clearer code structure
Add Accessibility Attributes
Always include alt attributes on images, lang attribute on html tag, proper heading hierarchy (h1 → h2 → h3), and ARIA labels where needed. These are required for WCAG 2.1 compliance.
Accessibility checklist: Alt text on images • Lang attribute • Proper headings • ARIA labels • Keyboard navigation
Include Essential Meta Tags
Add essential meta tags for SEO and functionality: charset, viewport, description, and Open Graph tags for social sharing. These improve SEO rankings and user experience.
Essential meta tags: charset="UTF-8" • viewport for mobile • description for SEO • og:tags for social
Validate Regularly
Validate your HTML code regularly—after major changes, before deployment, and as part of your build process. Use automated validation in CI/CD pipelines to catch errors early. Regular validation prevents issues from accumulating and becoming harder to fix.
Validation schedule: After code changes • Before deployment • In CI/CD pipeline • During code reviews
Frequently Asked Questions
How do I validate HTML code?
Paste your HTML code into the validator, click Validate, and review the results. The tool checks for syntax errors, missing tags, accessibility issues, SEO problems, and performance warnings. All validation happens locally in your browser for complete privacy.
What HTML errors does the validator detect?
Our HTML validator detects missing DOCTYPE, unclosed tags, mismatched closing tags, missing required elements (html, head, body, title), invalid attributes, and structural issues. It also checks for accessibility problems like missing alt text and SEO issues like missing meta tags.
Do you store my HTML code?
No. This HTML validator processes everything locally in your browser. Your code never leaves your device, ensuring complete privacy and security. No server uploads, no data storage, no privacy concerns.
What's the difference between errors and warnings?
Errors are critical issues that break HTML validity or functionality (missing closing tags, invalid structure). Warnings are important but non-critical issues (missing alt text, missing meta tags). Suggestions are best practices for better SEO, accessibility, and performance.
Does this validator check for accessibility issues?
Yes. Our HTML validator checks for accessibility issues including missing alt text on images, missing lang attribute, improper heading hierarchy, missing ARIA labels, and other WCAG compliance issues. This helps ensure your HTML is accessible to all users.
Can I validate HTML from a URL?
Yes. You can fetch HTML from a URL using the fetch feature, though it may be blocked by CORS policies. Alternatively, copy the HTML source code from your browser's developer tools and paste it into the validator for complete validation.
What SEO issues does the validator check?
The validator checks for missing meta description, missing Open Graph tags, improper heading hierarchy (h1 should be first, no skipped levels), missing title tag, and other SEO best practices. These checks help improve your search engine rankings.
Is this validator based on W3C standards?
Yes. Our HTML validator follows W3C HTML5 standards and checks for compliance with official HTML specifications. It validates syntax, structure, and best practices according to W3C guidelines and modern web standards.
How do I fix HTML validation errors?
Review the validation report to identify errors. Common fixes include: adding missing closing tags, fixing mismatched tags, adding required elements (DOCTYPE, html, head, body, title), correcting invalid attributes, and fixing structural issues. Our validator provides specific suggestions for each error to help you fix them quickly.
What's the difference between HTML validator and linter?
An HTML validator checks code against W3C standards for syntax errors and structural issues. An HTML linter focuses on code quality, style, and best practices. Our tool combines both—it validates HTML syntax and also checks for accessibility, SEO, and performance best practices, making it a comprehensive validation and linting solution.
Can I validate HTML5 code automatically?
Yes. You can integrate HTML validation into your development workflow using CI/CD pipelines, pre-commit hooks, or build tools. For quick validation, our free online HTML validator provides instant results without any setup. Validate HTML before deployment to catch errors early and maintain code quality.
Why is HTML validation important for SEO?
Valid HTML helps search engines understand and index your content better. Invalid HTML can cause parsing errors, missing meta tags, improper heading hierarchy, and accessibility issues—all of which can hurt SEO rankings. Valid HTML with proper semantic structure improves crawlability, indexing, and search engine visibility.
HTML Validation Guides
Step-by-step guides for validating HTML in different scenarios:
All HTML Validator guides (20 total)
Related HTML & Developer Tools
Explore our complete suite of developer tools for HTML and web development:
HTML Minifier
Compress HTML
Minify HTML code to reduce file size and improve performance after validation. Use our HTML Minifier to optimize validated code.
Open tool →
HTML Formatter
Beautify HTML
Format and beautify HTML code for better readability before validation. Our HTML Formatter helps prepare code for validation.
Open tool →
HTML Form Builder
Build Forms
Build accessible HTML forms, then validate the generated code. Check out our HTML Form Builder and HTML Tools collection.
Open tool →
Flags missing alt attributes on images
Validates ARIA role usage and attribute combinations
Identifies inputs without label associations
Free with no sign-up required
- Cost
- Free tier
- Sign-up
- Not required
- Processing
- Tool-specific
- Privacy
- Clearly disclosed
Add this HTML Validator to your website
Drop the HTML Validator into a blog post, product docs, intranet, or school portal with one iframe. Processing, privacy, and usage limits are the same as on the full tool page.
- One copy-ready line of HTML
- Responsive — adapts to any container width
- No API credentials are placed in the snippet
Embed code
<iframe
src="https://www.fixtools.io/html/html-validator?embed=1"
width="100%"
height="780"
frameborder="0"
style="border:0;border-radius:16px;max-width:900px;"
title="HTML Validator by FixTools"
loading="lazy"
allow="clipboard-write"
></iframe>Attribution-friendly: a small "Powered by FixTools" link appears in the embed footer.
What HTML Validation Catches for Accessibility and What Requires Additional Tools
The HTML specification defines a set of accessibility requirements that are testable at the markup level, which means they can be checked statically by a validator without needing to render the page or simulate user interaction. Missing alt attributes on img elements are spec violations because alt is a required attribute on img in nearly all cases, with the only exception being purely decorative images where an empty alt attribute (alt="") is required to signal that the image carries no content meaning. Invalid ARIA role values are spec violations because the WAI-ARIA specification defines a finite list of valid ARIA role values, and using a role name that does not exist in that list (whether through typo or fabrication) is an error that the validator can identify by matching against the known role list. ARIA attribute usage on elements that prohibit those attributes, such as aria-checked on a div that does not have a role permitting that state, is also a spec violation that validators report.
However, HTML validation does not cover the full scope of accessibility requirements defined by WCAG, the Web Content Accessibility Guidelines that form the basis of most accessibility law and policy. WCAG 2.1 Level AA, which is the most commonly cited compliance target, has fifty success criteria covering a wide range of accessibility concerns, and many of these criteria require human judgment or runtime testing rather than static markup analysis. Validation cannot check whether alt text is meaningfully descriptive rather than just present, whether color contrast meets the minimum required ratios for normal and large text, whether interactive elements are reachable and operable via keyboard, or whether the reading order produced for screen reader users makes logical sense. These checks require dedicated accessibility testing tools such as axe, WAVE, or Lighthouse, plus manual testing with an actual screen reader operated by either an accessibility specialist or an end user.
The value of HTML validation as an accessibility step is that it eliminates the base layer of structural errors before deeper accessibility testing begins, which makes the deeper testing more productive and its results easier to interpret. An accessibility audit that finds twenty issues on a page with invalid underlying HTML is significantly harder to interpret than one performed on a page that starts from a clean HTML baseline, because errors in markup structure, ARIA roles, and label associations compound each other in ways that obscure the root causes. Validating HTML first produces a cleaner starting point for accessibility testing and reduces the risk of false positives in automated accessibility scanners that are caused by structural markup issues rather than genuine accessibility failures.
A practical accessibility workflow combines validation with several other checks in a defined sequence. Step one is HTML validation to catch spec-level errors. Step two is an automated WCAG checker like axe DevTools or Lighthouse to surface contrast issues, missing form labels in context, and other rule-based accessibility problems. Step three is manual keyboard navigation testing to confirm that all interactive elements are reachable and operable without a mouse. Step four is screen reader testing with VoiceOver, NVDA, or another assistive technology to verify that the page is usable by people who depend on these tools daily. No single tool catches everything, and the combination of all four steps provides much better coverage than any one of them alone.
How to use this tool
Paste your HTML and validate. Pay close attention to errors about missing alt attributes, invalid ARIA roles, and inputs without associated labels. Fix all spec-level accessibility errors before running a full WCAG audit.
How It Works
Step-by-step guide to check html for accessibility errors:
- 1
Paste your HTML
Paste the full HTML source including all images, form elements, ARIA attributes, and interactive components. The validator needs the complete document context to apply accessibility-related rules correctly, particularly for ARIA attribute checking which depends on the surrounding role context.
- 2
Validate
Click the validate button to run the full spec-level accessibility check across all the rules the validator implements. The validator will scan every element and attribute and report any violations of the accessibility-relevant subset of HTML and ARIA spec rules.
- 3
Fix ARIA and alt text errors
Address missing alt attributes on images, invalid ARIA role names that do not appear in the WAI-ARIA spec, ARIA attributes used on elements that do not support them, and form inputs that are missing proper label associations. These are the most common accessibility-related errors that validation surfaces.
- 4
Revalidate
Confirm that all spec-level accessibility errors have been resolved by running another validation pass after your fixes. A clean validation result means you have a clean structural baseline ready for deeper accessibility testing with dedicated tools.
- 5
Run a full accessibility audit
With a clean HTML baseline in place, run an automated WCAG checker such as axe DevTools, WAVE, or Lighthouse to find accessibility issues that go beyond what HTML validation can detect, and then test the page manually with a screen reader to verify the experience for assistive technology users.
Real-world examples
Common situations where this approach makes a real difference:
Pre-audit accessibility preparation
Before submitting a site to a formal accessibility audit by an external consultant or compliance team, a developer validates all HTML to eliminate spec-level errors that would have been caught for free. This reduces the audit findings to genuine WCAG issues rather than structural HTML problems that should never have been part of the audit scope in the first place, making the audit significantly more efficient and its results more actionable for the development team to address.
Reviewing ARIA usage in a component library
A shared component library uses ARIA roles and attributes extensively for custom interactive widgets like dialogs, tabs, and combobox controls. Validating the HTML output of each component catches three invalid ARIA role names that were typos in the source code, two aria-* attributes used on elements that do not support them according to the WAI-ARIA spec, and a button element with a conflicting role attribute that was added redundantly during a refactor and never cleaned up.
Checking image alt text in a CMS-generated page
A content management system allows editors to insert images into articles without enforcing the requirement that alt text be provided, relying on editor discipline rather than tooling to ensure accessibility. Validating a page after editorial updates catches images added without any alt attribute at all, as well as images with empty alt attributes that should have had descriptive text because the images convey information rather than being purely decorative.
Accessibility QA for a redesigned form
A redesigned contact form is validated as part of the QA process before user testing begins. Validation reveals that three input elements lost their label associations during the redesign because the for and id attributes on the label and input elements were not updated to match the new structure, and that an ARIA live region uses an invalid aria-live attribute value that does not match any of the values defined in the WAI-ARIA spec. Both issues are fixed before users see the form.
Pro tips
Get better results with these expert suggestions:
Empty alt and missing alt are different errors
A missing alt attribute on an img element is always a spec error that validators flag because alt is a required attribute. An empty alt attribute (alt="") is the correct markup for decorative images that carry no content meaning, because screen readers skip elements with alt="" rather than announcing them. Using alt="" on images that actually convey information is an accessibility error that the HTML validator cannot detect because it only checks for presence of the attribute, not the appropriateness of its content. That deeper check requires human judgment or a dedicated audit tool.
ARIA roles override native HTML semantics
Adding role="button" to a div changes how assistive technologies treat the element, overriding the div's lack of inherent semantics with the explicit button role. However, ARIA roles do not add interactive behavior to the element: a div with role="button" must also handle keyboard events for Enter and Space key presses to be operable by keyboard users, and must manage focus correctly. The validator catches invalid role names but cannot check whether the required interactive behavior has been implemented in JavaScript, which is one of the most common accessibility failures in custom components.
aria-label and aria-labelledby can conflict
If both aria-label and aria-labelledby are present on the same element, the aria-labelledby attribute takes precedence according to the WAI-ARIA accessible name calculation algorithm. Having both attributes present is not a hard spec violation but is redundant and potentially confusing for anyone maintaining the code later. Validators may emit a warning about this combination. Use one approach consistently across your components rather than mixing both on the same element, and document the chosen convention in your component library style guide.
Validate before accessibility testing, not instead of it
HTML validation catches spec-defined accessibility errors only, which is a small subset of the full WCAG rule set. WCAG compliance requires testing color contrast ratios across all text and UI components, keyboard operability of every interactive element, reading order through the page, form error messaging patterns, focus management during dynamic content updates, and much more that no static validator can check. Treat spec validation as step one of a complete accessibility testing workflow, not as a substitute for the other steps in that workflow.
Frequently asked questions
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