HTML Validator

Check HTML for unclosed tags, mismatched nesting, duplicate IDs, and missing attributes.

HTML to validate

7 errors, 1 warning

errorline 9, col 3

Unclosed tag <p>

<p>This is a paragraph with an <a href="https://example.co

warningline 10, col 3

<img> missing recommended "alt" attribute

errorline 12, col 5

Unclosed tag <span>

<span>Nested content</div>

errorline 12, col 25

Mismatched tag: expected </span> (opened at line 12), found </div>

<span>Nested content</div>

errorline 13, col 3

Closing tag </div> has no matching open tag

</div>

errorline 14, col 3

Unclosed tag <p>

<p>Another paragraph

errorline 15, col 1

Mismatched tag: expected </p> (opened at line 14), found </body>

</body>

errorline 16, col 1

Mismatched tag: expected </p> (opened at line 14), found </html>

</html>

HTML validation and best practices

Valid HTML improves accessibility, search engine indexing, and cross-browser compatibility. This validator checks for structural issues: unclosed tags, mismatched nesting, duplicate IDs, and missing required attributes like alt on images. For full W3C validation, use the official Nu HTML Checker at validator.w3.org.

Built and maintained by Meet Shah · Last updated 19 July 2026

What this tool is used for

Finding an unclosed element that is making a layout collapse below a certain point.
Catching duplicate ids that break label associations and fragment links.
Checking a template's output after a change to its loops or conditionals.
Reviewing markup from a colleague when you cannot run a full validator.
Spotting mismatched nesting in a hand-edited fragment.

Frequently Asked Questions

Does invalid HTML break a page?: Rarely, because browsers implement a specified error-recovery algorithm — the parsing spec defines exactly what to do with a stray tag. That is why invalid markup usually renders, and also why it renders differently from what you intended rather than not at all.
What are the errors that actually matter?: Unclosed elements that swallow the rest of the document, duplicate IDs, and improperly nested block content inside inline elements. Those change the DOM tree, which changes what CSS selectors match and what assistive technology announces.
Why do duplicate IDs matter if the page still works?: Because `getElementById` returns the first, a `label` `for` attribute binds to the first, and an anchor link jumps to the first — so the second element becomes unreachable by every mechanism that relies on the ID. It is a silent accessibility failure.
Is a missing alt attribute a validation error?: Yes — `alt` is required on `img`, and omitting it is different from setting it empty. `alt=""` explicitly marks the image as decorative and tells a screen reader to skip it; a missing attribute leaves the reader to announce the filename instead.
Does valid HTML help SEO?: Not directly — Google parses broken markup the same way browsers do. What validity buys is that the DOM matches your intention, so headings nest correctly, structured data parses, and content does not end up inside an element that hides it.

Common errors and gotchas

Expecting full specification conformance, which needs the W3C validator rather than a structural check.
Assuming valid markup means accessible markup, which is a separate and larger question.
Validating a fragment as if it were a document, where missing head and body elements are expected.
Ignoring duplicate ids as cosmetic, when they break `aria-labelledby`, `for` and in-page links.
Validating server-templated source rather than the rendered output, which is what a browser sees.