🌐
W3C
validator.w3.org
The W3C Markup Validation Service
Check the markup (HTML, XHTML, ... Pack 2, see our information page on the W3C QA Website. ... This validator checks the markup validity of Web documents in HTML, XHTML, SMIL, MathML, etc....
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 Markup Validation Service
validator service by the World Wide Web Consortium
validator.w3.org
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 … Wikipedia
🌐 🌐
🌐
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.
Videos
🌐 youtube.com
How To Validate Your HTML5 Code Using An Online Validation Service ...
January 5, 2023
06:19
🌐 YouTube
How to use W3C HTML and CSS code validators - YouTube
November 23, 2025
05:55
🌐 YouTube
Using the W3C validator to check HTML - YouTube
September 10, 2024
16:17
🌐 YouTube
How To Validate HTML Code – What You Need To Know - YouTube
February 28, 2024
37
06:29
🌐 YouTube
W3C HTML Validator - Introduction & Compliance Benefits - YouTube
December 6, 2010
View all
View all
People also ask
Is W3C validation necessary for SEO?
W3C validation is not a direct ranking factor for SEO, but it helps ensure your website’s code is clean and error-free. This can lead to better browser compatibility, faster loading, and improved user experience, all of which support good SEO practices.
🌐
infidigit.com
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
Does W3C validation improve page speed?
While W3C validation itself doesn’t directly boost page speed, clean code can reduce rendering errors and improve browser efficiency. This can indirectly help your pages load faster and more smoothly.
🌐
infidigit.com
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
How do I know if my website passes W3C validation?
You can check your website’s W3C validation status using the W3C Markup Validation Service. Simply enter your website’s URL, and it will show you any coding errors or warnings that need to be fixed.
🌐
infidigit.com
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
🌐
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 › developers › tools
Validators and tools | Developers | W3C
This page lists the validators and web developer tools.
🌐
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.

Top answer
1 of 1
1
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.
🌐
Wikipedia
en.wikipedia.org › wiki › W3C_Markup_Validation_Service
W3C Markup Validation Service - Wikipedia
August 8, 2025 - Validation is instead performed using an open-source "Nu Validator", an instance of which is provided by W3C. The Markup Validation Service began as The Kinder, Gentler HTML Validator, a project by Gerald Oskoboiny.
HistoryBrowser accommodationLimitationsCSS validation
🌐
LoginRadius
loginradius.com › home
Curious About Clean Code? Discover Why W3C Validation Matters
January 26, 2015 - Validation is comparing your code to W3C standards. The best way to validate your code is by using the W3C validation tools. ... This validator checks the CSS validity of web documents in HTML, XHTML etc.
Find elsewhere
🌐
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 - Learn what W3C validation is and how it helps ensure a proper website structure. Check W3C HTML, CSS validators and fix markup issues for better SEO performance.
🌐
Chrome Web Store
chromewebstore.google.com › detail › w3c-markup-validation-ser › kfjemflfclknofbklcnbnpjelkfhcomk
W3C Markup Validation Service for CSR pages - Chrome Web Store
Learn more about results and reviews. ... Average rating 5 out of 5 stars. Learn more about results and reviews. Use W3C html validator online to validate html for errors and syntax issues.
🌐
Devzery
devzery.com › post › w3-website-validator-guide-to-html-and-css-validation
W3 Website Validator: Guide to HTML and CSS Validation
September 16, 2024 - Learn how to use the W3 Website Validator for error-free HTML and CSS. Improve your site's SEO, performance, and user experience.
🌐
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 - Streamlining Your Code: Top Online HTML Validators for Web Developers Here is a list of some popular online HTML validators that you can use to validate your HTML code: 1- W3C Markup Validation …
🌐
FreeFormatter
freeformatter.com › html-validator.html
Free Online HTML Validator - FreeFormatter.com
Validates HTML files for compliance against the W3C standards and performs linting to assess code quality against best practices.
🌐
GitHub
github.com › theoludwig › html-w3c-validator
GitHub - theoludwig/html-w3c-validator: CLI for validating multiple html pages using validator.w3.org. · GitHub
Quote from https://validator.w3.org/docs/help.html#why-validate: One of the important maxims of computer programming is: "Be conservative in what you produce; be liberal in what you accept." Browsers follow the second half of this maxim by accepting Web pages and trying to display them even if they're not legal HTML.
Starred by 5 users
Forked by 2 users
Languages   TypeScript 89.9% | HTML 9.4% | JavaScript 0.7%
🌐
npm
npmjs.com › package › w3c-html-validator
w3c-html-validator - npm
HTML validation messages containing any of the strings or matching any of the regexes will be skipped. Empty lines and lines starting with a hash sign (#) are treated as comments and do nothing. ... # Ignore Config for w3c-html-validator /^Duplicate ID/ /^Element .blockquote.
      » npm install w3c-html-validator
Published   Jan 21, 2026
Version   2.2.0
Author   Thomas Davis
Repository   https://github.com/center-key/w3c-html-validator
Homepage   https://github.com/center-key/w3c-html-validator
🌐
Tutorialspoint
tutorialspoint.com › home › html › html validation
HTML Validation - Enhance Your Coding Standards
February 6, 2007 - An HTML validator is a tool that parses each line of code in order to find syntax errors and to check whether the HTML and CSS codes comply with the latest standards set by the World Wide Consortium (W3C).
🌐
Stack Overflow
stackoverflow.com › questions › 20302686 › how-to-use-the-w3c-validator-api
How to use the w3c validator API
Top answer
1 of 2
8

Maybe this is a possible update:

In previous answers and comments only a SOAP API was mentioned, which is however, as described here, outdated.

A possibility to use the new(?) API is described here: 

https://validator.w3.org/nu/?doc=https%3A%2F%2Fwww.w3.org%2F&out=json

Maybe someone can confirm this.

2 of 2
0

Unfortunately, you have to set up a proxy to do this due the cross-domain restriction.

How to call SOAP WS from Javascript/jQuery

🌐
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?

Top answer
1 of 17
52
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.
2 of 17
12
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.
🌐
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%
🌐
Eniun
eniun.com › portada › html validation
HTML validation - Eniun
July 16, 2024 - There are several online tools and validators available that can help you validate your HTML code. One popular option is the W3C Markup Validation Service, which is provided by the World Wide Web Consortium (W3C).
🌐
Elegant Themes
elegantthemes.com › blog › wordpress › how to use w3c validator to improve seo
How to Use W3C Validator to Improve SEO
October 2, 2022 - The W3C validator checksHTML and CSS code for errors like missing alt tags and unclosed elements. Learn how to use it to boost your SEO!