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HTML Validator

Find unclosed tags, mismatched nesting, duplicate ids and missing alt text.

  • Free
  • No signup
  • Nothing uploaded
  • No watermark

9 errors, 1 warning

line 5</span> does not match <p> opened on line 5. Tags have to nest, not overlap.
line 6</div> does not match <p> opened on line 5. Tags have to nest, not overlap.
line 7Duplicate id "a" — also used on line 4. An id has to be unique; getElementById returns only the first.
line 7This <img> has no alt attribute. Screen readers announce the filename instead, and it fails every accessibility audit.
line 8</body> does not match <p> opened on line 5. Tags have to nest, not overlap.
line 9</html> does not match <p> opened on line 5. Tags have to nest, not overlap.
line 2<html> is never closed.
line 3<body> is never closed.
line 4<div> is never closed.
line 5<p> is never closed.

How to Use the HTML Validator Online

HTML Validator: before and after TEXT A YOUR INPUT HTML VALIDATOR DONE A RESULT

Finds unclosed tags, mismatched nesting, duplicate ids and missing alt text. Duplicate ids are the quiet one: the page still renders, and every script using getElementById silently takes only the first, which produces bugs that look random.

No software to install Free Works on any device

Step-by-Step Guide to Using the HTML Validator

  1. 1. Paste your code into the box

  2. 2. Problems appear underneath as you type, with the line each one is on

  3. 3. What the check does and does not cover is stated at the bottom — read it before trusting a clean result

Try it now

What is the HTML Validator?

Finds unclosed tags, mismatched nesting, duplicate ids and missing alt text. Duplicate ids are the quiet one: the page still renders, and every script using getElementById silently takes only the first, which produces bugs that look random.

Why Use Web Development Tools in the Browser?

Small web jobs like minifying CSS, checking HTML, escaping a string or writing a Dockerfile rarely deserve a new install. These tools do each one in a tab, and your code is never sent to a server.

“The fastest tool is the one already open in a tab.”
  • Code checked for errors
  • Messy code made readable
  • Number bases converted
  • Selectors and paths tested
  • Minified files for faster pages
  • Config files from templates
  • CI workflows drafted quickly
  • Source code stays private

Popular Uses for Developer Utilities

These are the small tasks that interrupt a day of web work.

  • Minifying assets Shrink CSS or JSON before deploying, then beautify it again to read.
  • Validating markup Catch unclosed tags and syntax errors in HTML, CSS or JavaScript before publishing.
  • Docker setups Draft a Dockerfile, Docker Compose file or .dockerignore for a new project.
  • CI pipelines Generate a GitHub Actions or GitLab CI workflow and adjust it to fit.
  • Number conversions Convert between binary, decimal, hex and octal while reading low-level data.
  • Selector testing Try a CSS selector or JSONPath expression against sample input and see matches.
  • Data URIs Turn a small image into a data URI to embed directly in CSS.
  • Project files Create .gitignore, EditorConfig and CODEOWNERS files without copying them from old repos.

What You Can Do with the HTML Validator

Understands HTML structure rather than treating the file as bytes

Validation happens on your own machine, at your machine's speed

Reports the line and column of the first real error rather than a generic parse failure

Expressions are tested against your own document, so the result is the one you will get

Why Use ToolXkit for HTML Validator?

Your files stay with you

Everything runs inside your browser. Nothing you open or type is uploaded to a server.

Web Dev Tools for Different Users

Front-end developers

Validate, minify and beautify HTML, CSS and JavaScript as part of daily work.

DevOps engineers

Draft Docker, cron and CI files quickly, then refine them in the repo.

Coding students

Learn number systems, escapes and selectors by trying examples and seeing results.

Technical writers

Escape code samples and check snippets before putting them into documentation.

QA testers

Write clear bug reports and test CSS selectors against real page markup.

Open source maintainers

Set up issue templates, CODEOWNERS and gitattributes for a shared project on GitHub.

Best Times to Validate or Minify Code

Keep these open in a tab when you are:

  • Starting a new repository
  • Preparing a site for launch
  • Tracking down a syntax error
  • Containerising an app
  • Setting up automated builds
  • Reading hex or binary values
  • Writing a bug report
  • Working on a locked-down laptop

Frequently Asked Questions

Answers to common questions about this tool.

My page looks fine in the browser but fails validation.

Browsers repair broken markup as they parse, so a missing closing tag usually renders correctly anyway. It still matters, because the repair is a guess — and screen readers, search crawlers and older browsers may guess differently. Validation errors are where those guesses can diverge.

Do these errors affect my SEO?

Most do not. Google parses forgivingly and ranks pages with imperfect markup every day. The ones that do matter are structural — an unclosed element that swallows the rest of the page, or a heading hierarchy the crawler reads differently from a reader.

Is my code sent to a server when I use HTML Validator?

No. Your code is processed in the browser tab, so private repositories, keys in config files and client work stay on your machine. It also means the tool works on locked-down work laptops where installing software is not allowed.

Will minifying break my code?

Minifying removes spaces, comments and line breaks without changing what the code does. Keep an unminified copy for editing, and test the minified version before deploying it.

Are the generated config files ready to use?

They give you a sound starting point based on common settings. Every project is different, so read through the file and adjust paths, versions and secrets before committing it.

Is HTML Validator free with no account?

Yes. There is no login, API key or usage cap. Open the page whenever you need it, even on a machine that is not your own.

Does HTML Validator follow current web standards?

The validators and generators follow current web standards and common tool conventions. Standards keep moving, so for edge cases check MDN or the official specification listed in further reading.

Further reading

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