W3C
validator.w3.org
The W3C Markup Validation Service
W3C's easy-to-use markup validation service, based on SGML and XML parsers.
W3 Nu Html Checker
This tool is an ongoing experiment in better HTML checking, and its behavior remains subject to change · Show sourceoutlineimage reporterrors & warnings onlycheck error pagesUser-Agent Accept-Language
W3C Feed Validation Service, for Atom and RSS
This is the W3C Feed Validation Service, a free service that checks the syntax of Atom or RSS feeds.
w3 Validator
This Link Checker looks for issues in links, anchors and referenced objects in a Web page, CSS style sheet, or recursively on a whole Web site. For best results, it is recommended to first ensure that the documents checked use Valid (X)HTML Markup and CSS. The Link Checker is part of the W3C's ...
W3C
jigsaw.w3.org › css-validator
The W3C CSS Validation Service
Check Cascading Style Sheets (CSS) and (X)HTML documents with style sheets ... Note: If you want to validate your CSS style sheet embedded in an (X)HTML document, you should first check that the (X)HTML you use is valid.
Is HTML validation still a useful service, or is it obsolete by now?
There are better ways to do this now. You can use linters make sure your html is accessible. You can use a build system to make sure your js and css work on whatever browser’s you target. Or you can just configure a linter to make sure you’re only using features supported by the browsers you are targeting. Lighthouse and similar programs can verify performance and accessibility. More on reddit.com
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March 22, 2024W3C Validator alternatives for broken HTML?
Which of w3 validators do you use? What doctype is set on your documents? Give some examples of "valid things" that show errors in validator? More on reddit.com
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May 9, 2025java - How to validate that HTML matches W3C standards - Stack Overflow
I have a project that generates HTML pages using a Velocity template and Java. But most of the pages do not comply with W3C standards. How can I validate those HTML pages and get a log telling me w... More on stackoverflow.com
W3C HTML validation for React JSX files
Reactjs When working on front-end projects I always like to work with linting tools. They prevent from dumb mistakes to serious smelly code pieces … More on reddit.com
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npm
npmjs.com › package › w3c-html-validator
w3c-html-validator - npm
January 21, 2026 - Check the markup validity of HTML files using the W3C validator. Latest version: 2.2.0, last published: 2 months ago. Start using w3c-html-validator in your project by running `npm i w3c-html-validator`. There are 4 other projects in the npm registry using w3c-html-validator.
» npm install w3c-html-validator
Published Jan 21, 2026
Version 2.2.0
Author Thomas Davis
W3Schools
w3schools.com › w3css › w3css_validation.asp
W3.CSS Validation
The W3C CSS Validator checks for CSS1, CSS2, CSS3, and CSS4 properties.
Reddit
reddit.com › r/webdev › is html validation still a useful service, or is it obsolete by now?
r/webdev on Reddit: Is HTML validation still a useful service, or is it obsolete by now?
March 22, 2024 -
It's been 20 years since I learned the bulk of my web dev skills, and I'm very much stuck in my old ways. Every so often I realize I've been wasting time doing something the hard way when there's no need for it anymore, it's just what is familiar.
The main point of validating HTML/CSS back in the day was to ensure your site worked/looked the same regardless of which browser it was accessed with. Web standards have developed enough by now that it's no longer an issue, rather the focus now is on various devices/screen sizes.
(If anyone has no clue what I'm talking about: https://validator.w3.org/)
Does validation serve much purpose beyond double-checking your syntax? Is there a better way to do that when writing a page by hand?
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There are better ways to do this now. You can use linters make sure your html is accessible. You can use a build system to make sure your js and css work on whatever browser’s you target. Or you can just configure a linter to make sure you’re only using features supported by the browsers you are targeting. Lighthouse and similar programs can verify performance and accessibility.
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It can be good to double check your output. Maybe your CMS or other parts generate HTML which is not valid. And validity is still something you should aim for.
Wikipedia
en.wikipedia.org › wiki › W3C_Markup_Validation_Service
W3C Markup Validation Service - Wikipedia
August 8, 2025 - The Markup Validation Service is a validator by the World Wide Web Consortium (W3C) that allows Internet users to check pre-HTML5 HTML and XHTML documents for well-formed markup against a document type definition (DTD).
Chrome Web Store
chromewebstore.google.com › detail › w3c-markup-validation-ser › kfjemflfclknofbklcnbnpjelkfhcomk
W3C Markup Validation Service for CSR pages - Chrome Web Store
A tool for validating the currently rendered html with the Markup Validation Service provided by the W3C. It provides the ability to extract the HTML of the current page and populate values into Validate by Direct Input in the Markup Validation ...
GitHub
github.com › theoludwig › html-w3c-validator
GitHub - theoludwig/html-w3c-validator: CLI for validating multiple html pages using validator.w3.org. · GitHub
CLI for validating HTML pages using validator.w3.org. html-w3c-validator is a CLI tool to validate HTML pages using validator.w3.org.
Starred by 5 users
Forked by 2 users
Languages TypeScript 89.9% | HTML 9.4% | JavaScript 0.7%
W3C
w3.org › developers › tools
Validators and tools | Developers | W3C
This page lists the validators and web developer tools.
Reddit
reddit.com › r/webdev › w3c validator alternatives for broken html?
r/webdev on Reddit: W3C Validator alternatives for broken HTML?
May 9, 2025 -
I've always used the W3C Validator to help find broken HTML elements, but I'm finding it's becoming quite outdated and throwing errors for things that are now valid.
Are there any better alternatives to finding broken HTML elements?
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Which of w3 validators do you use? What doctype is set on your documents? Give some examples of "valid things" that show errors in validator?
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The main W3C validator has gone horrifically out of date. WHATWG recommends using the Nu HTML Checker from W3C instead: https://validator.w3.org/nu/ Nothing is more reliable than putting the HTML into a variety of web browsers and checking the console output, though.
GitHub
github.com › w3c › markup-validator
GitHub - w3c/markup-validator · GitHub
The tool consists of a perl-based CGI script that uses DTD to verify the validity of HTML3, HTML4 and XHTML documents; it also incorporates by reference the NU Validator used to validate HTML5 / HTML LS documents. W3C Software License and Notice.
Starred by 137 users
Forked by 32 users
Languages HTML 79.1% | Perl 13.3% | Python 2.2% | CSS 2.1% | Clean 1.3% | JavaScript 0.9%
W3C
w3.org
W3C
The World Wide Web Consortium (W3C) develops standards and guidelines to help everyone build a web based on the principles of accessibility, internationalization, privacy and security.
Medium
medium.com › @gulshan_n › streamlining-your-code-top-online-html-validators-for-web-developers-49182f1e2d5c
Streamlining Your Code: Top Online HTML Validators for Web Developers | by Gulshan N | Medium
September 18, 2023 - Description: Provided by the World Wide Web Consortium (W3C), this validator checks your HTML code against the official HTML specifications and standards.
Eniun
eniun.com › portada › html validation
HTML validation - Eniun
July 16, 2024 - You can access the service at validator.w3.org and submit your HTML code for validation. To validate your HTML using the W3C Markup Validation Service, follow these steps:
Packagist
packagist.org › packages › glicer › w3c-validator
glicer/w3c-validator - Packagist.org
Validate html and css files using w3c markup validation and w3c css validation.
Infidigit
infidigit.com › home › technical seo › what is w3c validation? a guide to html, css & markup validators
What is W3C Validation: How the W3C Validator Works with HTML & CSS
September 24, 2025 - Here is a detailed explanation of how it works: Simply copy and paste the URL into the address bar to validate your page URL. Then, tap the check button, and the tool will begin validating the code.
University of Exeter
newton.ex.ac.uk › validator › htdocs › docs › users.html
User Documentation for The W3C MarkUp Validation Service
W3C's easy-to-use HTML validation service, based on an SGML parser.
Visual Studio Marketplace
marketplace.visualstudio.com › items
W3C Offline HTML Validator - Visual Studio Marketplace
October 25, 2025 - Extension for Visual Studio Code - A VSCode extension for offline HTML validation using W3C standards.
W3C
w3.org › WAI › WCAG21 › Techniques › general › G134
G134: Validating web pages | WAI | W3C
The W3C Markup Validation Service by the World Wide Web Consortium allows you to validate HTML files by URI, by file upload and by direct input of complete HTML documents.
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You can use the W3C validator directly from Java, see w3c-jabi.
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There is also an experimental API available from W3C to help automate validation. They kindly ask that you throttle requests, and also offer instructions on setting up a validator on a local server. It's definitely more work, but if you're generating a lot of HTML pages, it would probably make sense to also automate the validation.
http://validator.w3.org/docs/api.html