About HTML Validator

Validate HTML markup and structure. Part of the DevTools Surf developer suite. Browse more tools in the HTML collection.

Use Cases

• Frontend devs ensuring cross-browser compatibility
• SEO specialists fixing markup errors that hurt rankings
• Accessibility engineers validating semantic HTML structure
• QA teams running automated HTML checks in CI pipelines

Tips

• Fix errors from top to bottom as early errors can cascade
• Pay attention to accessibility warnings about ARIA attributes
• Validate after every major template change to catch regressions

Fun Facts

• The W3C Markup Validation Service was first launched in 1994 and has validated billions of documents since.
• HTML5 relaxed validation rules significantly, allowing unclosed <p> and <li> tags as valid markup.
• A 2023 HTTP Archive study found that the average web page contains 25 HTML validation errors.

FAQ

What does 'valid' HTML mean?

Conformance to the HTML5 spec: proper nesting, required attributes, no duplicate IDs, matching open/close tags. The validator reports the exact line and column of each issue.

Does it check accessibility?

Only the bits that overlap with validity — missing alt text, missing label associations, invalid aria values. Use a dedicated accessibility tool (axe, WAVE) for a full audit.

Will it flag custom elements?

Custom elements (hyphenated tags like `<my-widget>`) are allowed by spec — the validator recognizes them. Unknown standard-looking tags (`<mydiv>`) are flagged.

Can I validate HTML fragments?

Yes — the validator auto-wraps fragments in a minimal document. Paste a snippet from a template or a CMS without building a full page first.