W3C
validator.w3.org
The W3C Markup Validation Service
... This validator checks the markup validity of Web documents in HTML, XHTML, SMIL, MathML, etc. If you wish to validate specific content such as RSS/Atom feeds or CSS stylesheets, MobileOK content, or to find broken links, there are other validators and tools available.
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 · Enter the URI of a document (HTML with CSS or CSS only) you would like validated:
Videos
How To Validate Your HTML5 Code Using An Online Validation Service ...
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HTML: Validating code to HTML5 standards at validator.w3.org - YouTube
06:06
Validating HTML5 - YouTube
16:17
How To Validate HTML Code – What You Need To Know - YouTube
01:49
Using the W3C Markup Validation Service to check your HTML - YouTube
04:00
Validation of HTML document using validator.w3.org - YouTube
html - Add W3C HTML5 Validator to website - Stack Overflow
Does anybody know how to add the W3C HTML 5 validator onto my website.(So that I have a direct link to the validator which checks my page) Thanks More on stackoverflow.com
HTML 5 Validation Service - Stack Overflow
You can ask that W3C validator to check against e.g. HTML 4.01 Strict, which does show errors. Just pick the appropriate doctype on validator.w3.org/#validate_by_input+with_options. ... Thanks, True That. That does the trick for html 4. I have changed the title - I need to check html5 files. More on stackoverflow.com
What is the W3C VALIDATOR?
The w3c validator is one of the many HTML validators that checks your HTML markup to see if you are following the rules for constructing proper HTML. What is Markup Validation? Most pages on the World Wide Web are written in computer languages (such as HTML) that allow Web authors to structure text, add multimedia content, and specify what appearance, or style, the result should have. As for every language, these have their own grammar, vocabulary and syntax, and every document written with these computer languages are supposed to follow these rules. The (X)HTML languages, for all versions up to XHTML 1.1, are using machine-readable grammars called DTDs, a mechanism inherited from SGML. However, Just as texts in a natural language can include spelling or grammar errors, documents using Markup languages may (for various reasons) not be following these rules. The process of verifying whether a document actually follows the rules for the language(s) it uses is called validation, and the tool used for that is a validator. A document that passes this process with success is called valid. With these concepts in mind, we can define "markup validation" as the process of checking a Web document against the grammar (generally a DTD) it claims to be using. Web browsers are actually very tolerant of mistakes and try to render the HTML as best it can no matter how broken it is. For example, you can create a page missing several closing tags and it will still render something. So just because the HTML renders in the browser doesn't mean all is well. There are no rules as to how invalid HTML should be handled, which is why every browser handles these mistakes in different ways. So it's important to always try and serve up valid HTML. More on reddit.com
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January 7, 2023W3C 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, 2025GitHub
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%
Stack Overflow
stackoverflow.com › questions › 22129931 › add-w3c-html5-validator-to-website
html - Add W3C HTML5 Validator to website - Stack Overflow
<script type="text/javascript"> window.onload =function () { var submitbutton = document.getElementById("q"); if (submitbutton.addEventListener) { submitbutton.addEventListener("click", function () { if (submitbutton.value == 'Validate') { submitbutton.value = ''; } }); } } </script> <div id="header"> <form action="http://validator.w3.org/check" id="newsearch" method= "get" name="newsearch"> <input class="textinput2" id="q" maxlength="120" name="uri" placeholder="Enter URL" size="21" type="text"> <input class= "button2" type="submit" value="Validate"> </form> <div class="clear"></div> </div>
Wikipedia
en.wikipedia.org › wiki › W3C_Markup_Validation_Service
W3C Markup Validation Service - Wikipedia
August 8, 2025 - In November 2008, the W3C released the validator.nu HTML5 engine and the ability to check documents for conformance to HTML5.
W3C
w3.org › developers › tools
Validators and tools | Developers | W3C
This page lists the validators and web developer tools.
Appspot
h5validator.appspot.com › dcm
HTML5 Validator
Privacy Policy - Terms of Service · Feedback · feedback
Reddit
reddit.com › r/learnprogramming › what is the w3c validator?
r/learnprogramming on Reddit: What is the W3C VALIDATOR?
January 7, 2023 -
Hello this is willijkm speaking. actually i read a book about html and in a chapter of this book speak about W3C VALIDATOR but i don't quiet understand it, Can you help me? Thanks.
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.
W3C
w3.org › blog › 2008 › w3c-validator-now-with-html5
W3C Validator, now with HTML5 flavour | 2008 | Blog | W3C
The solution, indeed, is in resisting the “not invented here” syndrome, and embracing great projects, whether they are developed within W3C or by someone in the Web Community. We already provided a home for the feed validator. And yesterday, we launched a new version of the markup validator which integrates with the validator.nu engine for HTML5 support.
Raymii
raymii.org › s › tutorials › Local_W3C_HTML5_Validator_Server.html
Local W3C HTML5 Validator Server - Raymii.org
This tutorial shows you how to set up a local instance of the w3c html validator, including HTML5 validation support via a local instance of the Validator.nu HTML5 validator. The online w3 validator has strict limits and will ban you for some time if you validate to often.
Docker Hub
hub.docker.com › r › heitorganzeli › docker-w3c-html5-validator
heitorganzeli/docker-w3c-html5-validator - Docker Image
docker-w3c-html5-validator · © 2025 Docker, Inc. All rights reserved. | Terms of Service | Subscription Service Agreement | Privacy | Legal
Validator
about.validator.nu
About Validator.nu
fantasai designed the (X)HTML5 schema framework, wrote the (X)HTML5 Core schemas and helped along the way when I added features. JavaScript bits, the favicon and a lot of bug reports were contributed by Simon Pieters. The schemas for RELAX NG and XSLT were written by James Clark. The principal author of the schema for DocBook is Norman Walsh. The SVG schemas come from the W3C.
Packagist
packagist.org › packages › glicer › w3c-validator
glicer/w3c-validator - Packagist.org
"/glicer.html"]; //return array of reports path in html format or null if ok $results = $validator->validate( $files, ['html', 'css'], //validate html and css files function (SplFileInfo $file) { //callback function echo $file->getRealpath(); } ); var_dump($results); $results is an array that associate values to keys : filename checked => W3C html report or null if no error · In this example, you can view reports in result/w3c_css_glicer.html, result/w3c_html_glicer.html, result/... from your browser. Docker must be installed · docker pull magnetikonline/html5validator docker run -d -p 8080:80 -p 8888:8888 magnetikonline/html5validator ·
W3C
w3.org › blog › International › 2016 › 07 › 13 › w3c-html5-validator-enhanced-with-language-detection-functionality
W3C HTML5 Validator enhanced with language detection functionality | Internationalization Activity Blog
The W3C HTML5 Validator has been enhanced with functionality that detects the overall language of a page.
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:
Quora
quora.com › How-do-you-validate-a-website-for-HTML-5-compliance
How to validate a website for HTML 5 compliance - Quora
Answer (1 of 3): Html can be validated with a lot of tools these days here is a link through which you can validate your html :Markup Validation Service Html (hyper text markup language) is a far more forgiving language in terms errors and warning if the syntax is not correctly written the html...
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 ...