HTML Validator
Validate HTML markup - find unclosed tags, missing alt text, heading hierarchy and accessibility issues.
Validate HTML markup - find unclosed tags, missing alt text, heading hierarchy and accessibility issues.
This free HTML Validator from KX Toolkit is part of our all-in-one online toolkit. It runs entirely in your browser, so your data never leaves your device for client-side operations. 100% free, forever - no paywall, no credit card, no trial.
How to use the HTML Validator
- Paste your input - JSON, regex pattern, JWT, URL etc.
- Pick any flags or options the tool supports.
- Click the action button (Format, Test, Decode).
- Copy the result or download it as a file.
What you can do with the HTML Validator
- Format and validate API responses while debugging.
- Test regex patterns against real input before deploying.
- Decode JWTs to inspect claims and expiry.
- Generate UUIDs for migrations, tests and seeders.
Why use KX Toolkit's HTML Validator
- Browser-based: Works on Windows, macOS, Linux, iOS and Android - no install, no extension.
- Privacy-first: Client-side tools never upload your data; server-side tools delete files right after processing.
- Mobile-friendly: Full feature parity on phones and tablets - not a stripped-down view.
- Fast: Optimised for instant feedback. No artificial waiting screens, no email-gated downloads.
- One hub for everything: 300+ tools across SEO, text, image, PDF, code, color, calculators and more - skip switching between sites.
Tips for the best results
Bookmark the most-used tools - your browser bookmark bar is faster than retyping the URL every time.
Related Developer Tools
If you find this tool useful, explore the full Developer Tools collection or browse our complete tool directory. KX Toolkit is built for marketers, developers, designers, students and anyone who needs a quick utility without signing up for yet another SaaS.
What kinds of issues does the validator find?
Common findings include unclosed or mismatched tags, missing alt text on images, missing labels on form fields, duplicate id values, skipped heading levels, deprecated tags like center or font, missing required attributes, and invalid nesting like a div inside a p. Some validators also flag accessibility heuristics that go beyond strict HTML conformance.
Why is missing alt text such a common warning?
Alt is required on every img to satisfy WCAG and the HTML spec, even if the value is an empty string for decorative images. Screen readers fall back to reading the filename when alt is absent, which is rarely useful. Setting alt is the single highest-leverage accessibility fix in most codebases because it is mechanical to add and improves real user experience.
Does invalid HTML break my site?
Often it renders fine because browsers are extremely forgiving. The deeper costs are subtle: assistive technologies misinterpret the structure, CSS selectors target the wrong elements after the parser auto-corrects them, and JavaScript tooling like web scrapers or test runners produce flaky results. Validation is a quality investment that pays off in maintainability and accessibility.
How is the validator different from a linter?
A validator checks against the HTML spec - does the document parse correctly and follow content-model rules. A linter goes further and enforces stylistic and best-practice rules like attribute order, quote consistency, and required ARIA attributes. Run both: validate for correctness, lint for consistency. They overlap on accessibility checks.
What about heading hierarchy warnings?
Skipping levels - going from h1 directly to h3 - confuses screen readers that build an outline from headings. The validator flags it because users navigate by heading and missing levels feel like missing chapters. Use the heading order that reflects content structure; if you only need styled text, change the visual size with CSS instead of jumping levels.
Will the validator catch all accessibility issues?
No. It catches structural issues that imply accessibility problems but cannot evaluate things like color contrast, keyboard focus order, or live-region announcements without rendering and interaction. Pair the validator with axe, WAVE, or Lighthouse for a fuller picture, and ultimately test with real assistive technology like VoiceOver or NVDA.
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