HTML Validator

Catch broken markup before it ships

Missing a single </div> or crossing two tags can push a whole layout off, break a component, or confuse a scraper — and the browser silently "fixes" it, so the mistake never shows up until something downstream breaks. This validator scans your HTML and reports exactly what is wrong and where, so you can fix it at the source.

Paste a full page or a single snippet, and every issue comes back with its line and column and a plain-language explanation of what was expected.

What it checks

• Unclosed tags — an element that is opened but never closed by the end of the input (<div><p>text → <p> and <div> are never closed).
• Nesting issues — overlapping / misnested tags where a closing tag crosses another still-open element (<b><i>hi</b></i>), and stray closing tags with no matching open (</span> on its own).
• Syntax errors — an unterminated tag with no closing >, an unterminated <!-- … --> comment, and tags with no name.

It understands void elements (br, img, hr, input, meta, link, …) that never take a closing tag, self-closing tags (<hr/>), quoted attribute values (so a > inside alt="a > b" is safe), and the raw contents of <script>, <style>, <textarea>, and <pre> — a < inside JavaScript or text is not treated as a tag.

Worked example

Input:

<div><p>Hello<span>world</div>

Report:

Invalid HTML: 3 error(s), 0 warning(s) in 0 element(s).

  error   line 1:6   `<p>` (opened at line 1:6) is not closed before `</div>` — overlapping/misnested tags
  error   line 1:14  `<span>` (opened at line 1:14) is not closed before `</div>` — overlapping/misnested tags
  error   line 1:1   `<div>` is never closed with `</div>`

Output formats

Choose report for a readable, line-numbered issue list, or json for a machine-readable { valid, errors, warnings, elements, issues[] } object you can drop into a test, a CI check, or an editor plugin. Each issues[] entry carries severity, line, column, and message.

Privacy

Everything runs locally in your browser through WebAssembly. Your HTML is never uploaded to a server.

FAQ