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HTML Validator

Catch HTML mistakes before they ship. Paste your markup to find unclosed tags, mismatched nesting, duplicate IDs, images missing alt text and deprecated tags. Free, instant, private.

Validate HTML

Paste your markup. Issues are listed with line numbers.

Results

Paste HTML and press Validate.

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What it checks

The validator parses your markup for structural problems: tags that were never closed, closing tags with no opener, crossed nesting, void elements with closing tags, duplicate id attributes, images missing alt, links missing href and long-deprecated tags like <font> and <marquee>.

Errors break rendering or behavior; warnings are quality and accessibility smells worth fixing.

Honest limits

This is a fast structural check, not the full W3C validator: it does not verify attribute values, ARIA roles, or conformance to a specific HTML version. For spec-level validation, run the W3C validator as well.

Everything runs locally, so you can validate private templates and drafts without publishing them anywhere.

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HTML Validator FAQs

What is the difference between an error and a warning?
Errors are structural breaks: unclosed tags, stray closers, duplicate IDs. Warnings are quality issues: missing alt text, missing doctype, deprecated tags. Fix errors first.
Why does it flag my <img> tag?
Every image needs an alt attribute for accessibility and for cases where the image fails to load. Use alt="" for purely decorative images so screen readers skip them.
Is a duplicate ID really a problem?
Yes. IDs must be unique per document; duplicates break getElementById, label associations, fragment links and CSS targeting in unpredictable ways.
Does this replace the W3C validator?
No. This catches the structural mistakes that break pages day to day. The W3C validator additionally checks attribute values and spec conformance, so use both for important pages.
Is my HTML uploaded?
No. Validation runs entirely in your browser with plain JavaScript. Your markup never leaves your device.
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