HTML Validator
Catch unclosed, mismatched and stray HTML tags with a fast structural check. It runs entirely on your device — nothing is uploaded.
Example
Paste HTML to spot unclosed or mismatched tags. This is a fast structural check, not a full W3C conformance audit.
How it works
The validator tokenises your tags and tracks nesting (ignoring void elements like
and ![]()
), flagging tags that are never closed or closed out of order.
Good to know
The HTML Validator is a structural tag checker for developers, students, and content editors who need to confirm that an HTML snippet or page opens and closes its tags correctly. You paste markup into the box and it parses the tag stack in your browser, surfacing tags that are never closed, tags that close in the wrong order, and stray closing tags that have no matching opener.
Reach for it when something renders oddly, when a layout "leaks" past where it should stop, or when copied markup from a CMS, email template, or AI-generated block behaves unpredictably. A single missing </div> can cascade into broken styling for everything below it, and that root cause is exactly the kind of thing this check isolates quickly without you reading every line by hand.
Read the results as a nesting report, not a style grade. It treats void elements like <br>, <img>, <input>, and <hr> as self-completing, so it won't complain that they lack a closing tag. When it flags a problem, work from the first reported error downward, because one unclosed parent often produces a chain of misleading follow-on warnings that vanish once the original tag is fixed.
One caveat: this is a fast structural pass, not a full conformance audit. It checks that your tag tree is balanced, but it does not verify the W3C specification, attribute correctness, accessibility, or whether an element is legal inside its parent. For shipping a public page, use it to clear obvious nesting bugs first, then run a formal validator and an accessibility check before release.
Frequently asked questions
Is my data uploaded anywhere?
No — everything runs in your browser. Your code never leaves your device, so it's safe for private work and runs offline once loaded.
Is this tool free?
Yes, completely free with no sign-up and no limits.
People also ask
What does it mean when an HTML tag is unclosed?
An unclosed tag is an opening tag, such as <div>, that has no matching </div> before the document ends or before its parent closes. Browsers try to auto-correct it, but the guessed structure often differs from what you intended, causing layout and styling to break.
What are void elements in HTML?
Void elements are tags that cannot contain content and have no closing tag, including br, img, input, hr, meta, and link. A structural validator treats them as complete on their own, so it will not report them as missing a closing tag.
Why does my page layout break from one missing closing tag?
When a container like a div is left open, the browser keeps nesting everything after it inside that container. This shifts widths, padding, and positioning for all following content, so a single error can appear to break an entire section of the page.
Is structural HTML validation the same as W3C validation?
No. Structural validation checks only that tags are balanced and nested correctly, while the W3C validator checks full spec conformance, including allowed attributes, element placement rules, and document type requirements. Structural checks are faster but less thorough.
Can a browser fix invalid HTML automatically?
Browsers follow the HTML parsing standard to recover from many errors, such as silently closing unclosed tags. However, the recovery is a best guess and is not consistent across all cases, so relying on it can produce results that differ from your intent.
What is a mismatched HTML tag?
A mismatched tag occurs when elements close in the wrong order, for example <b><i>text</b></i> instead of <b><i>text</i></b>. The nesting overlaps rather than properly containing each element, which browsers handle inconsistently.
How do I find which HTML tag is causing a problem?
Start from the first reported error and fix it before judging the rest, since one unclosed or mismatched tag commonly triggers several follow-on warnings. Re-checking after each fix usually clears the chain of secondary errors.
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