HTML Tools
HTML Validator
Catch the unclosed <div> before it breaks your layout — structural validation with line numbers, instantly.
In short
Validate HTML online: find unclosed elements, mismatched tags, and misused void elements with line numbers — instant structural checking, in-browser.
By NaveenKumar T · Updated
- Unclosed-tag detection
- Mismatched-nesting checks
- Void-element rules enforced
Runs entirely in your browser — nothing you paste or open here is uploaded, logged, or stored.How we handle data →
Example
HTML Validator: input and output
A signup form that passes: void elements without closers, a comment and a script string that both contain tags.
<form id="signup">
<!-- TODO: remove the old <div> wrapper -->
<label>Email <input type="email" name="email" required></label><br>
<img src="/badge.svg" alt="">
<button type="submit">Join</button>
</form>
<script>document.getElementById("note").innerHTML = "<b>Saved</b>";</script>✓ No structural problems found
Tags are balanced, void elements are used correctly, and raw-text sections are well-formed.The <div> inside the comment and the <b> inside the script string were both ignored — comments are blanked and raw-text blocks skipped — and <input>, <br> and <img> passed without closing tags because they are void elements.
Learn more
Why browsers hide your HTML errors
Browsers error-correct aggressively: an unclosed <div> doesn't fail, it silently re-parents the rest of the page — and your CSS selector or querySelector stops matching with no error anywhere. Validation makes those silent repairs visible, pointing at the exact line where the structure went wrong.
The checker understands HTML's quirks: void elements never close, </script> inside strings doesn't fool it, comments and CDATA are skipped. What's flagged is what genuinely matters.
The bugs structural validation catches
A field guide from real debugging sessions: a div closed twice makes everything after it a child of <body>, so a sticky footer overlaps content; a <p> wrapping a <div> gets auto-closed by the parser, so the CSS targeting 'p > div' silently matches nothing; an unclosed <a> turns half the page into one giant link; a <li> outside any list renders but breaks keyboard navigation. None of these throw errors anywhere — the page just behaves wrong. Structural validation surfaces them as a list with line numbers, typically faster than bisecting the page by deletion.
Run it before publishing CMS content too: rich-text editors are prolific producers of mis-nested markup, and one pass here catches what the WYSIWYG view hides.
Help
Frequently asked questions
Everything you need to know about the HTML Validator.
01How do I use the HTML Validator?
Paste your HTML and press Validate. The checker walks the tag structure — respecting void elements like <br> and raw-text blocks like <script> — and reports the first structural problem with its line number.
02Is this the same as the W3C validator?
It's the structural layer of it: tag balance, nesting, void-element misuse — the errors that actually break layouts and JavaScript. Full W3C conformance (attribute vocabularies, ARIA rules, obsolete elements) requires their service; for everyday 'why is my page broken' debugging, structure is what you need first.
03Why does it flag a <li> or <p> I never closed, when browsers accept that?
Because the checker requires every non-void element to be closed explicitly. HTML's optional-end-tag rules — <p>, <li>, <tr>, <td>, <option> may end implicitly — are not modelled, so an omitted closer is reported as an unclosed element at the line where it opened. That strictness is deliberate: explicit closing is what formatters, XML pipelines and most linters expect, and omitted tags are exactly where mis-nesting hides. Only the first problem is reported; fix it and validate again to reveal the next.