Help and FAQ for the Markup Validator
Nothing wrong with the validator here, it just knows HTML better than you do. -- David Dorward, Validator's mailing-list.
Table of contents
- About the Markup Validator
- Using this service
- Miscellaneous (Very) Frequently Asked Questions
About the Markup Validator
Help me! I clicked on an icon and ended up on this strange site!
The author of the Web page you come from once used our service to validate that page, and the page passed validation. The author was then authorized to use the icon on that page, as a claim of validity. The icon is used as a link back to the validation service, so that the author can revalidate whenever necessary. This is why, by clicking on the icon, you followed a link to the current validation results for the page you came from.
The validation result was certainly positive ("this page is valid..."), but if it wasn't, you would probably do the author of the page where the icon was a favor if you could warn him/her of this abnormal situation.
If you are curious about Markup validation you may read this help document further, or you may simply use the back button of your Web browser to come back to the page where you found the "valid" icon.
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. 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.
Is validation some kind of quality control? Does "valid" mean "quality approved by W3C"?
Validity is one of the quality criteria for a Web page, but there are many others. In other words, a valid Web page is not necessarily a good web page, but an invalid Web page has little chance of being a good web page.
For that reason, the fact that the W3C Markup Validator says that one page passes validation does not mean that W3C assesses that it is a good page. It only means that a tool (not necessarily without flaws) has found the page to comply with a specific set of rules. No more, no less. This is also why the "valid ..." icons should never be considered as a "W3C seal of quality".
What is the Markup Validator and what does it do?
The Markup Validator is a free tool and service that checks the syntax of (X)HTML documents.
The Validator is sort of like
lint for C. It compares your HTML document to the defined syntax of HTML and reports any discrepancies.
Learn more about the Markup Validator.
Why should I validate my HTML pages?
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. Usually this means that the browser will try to make educated guesses about what you probably meant. The problem is that different browsers (or even different versions of the same browser) will make different guesses about the same illegal construct; worse, if your HTML is really pathological, the browser could get hopelessly confused and produce a mangled mess, or even crash.
That's why you want to follow the first half of the maxim by making sure your pages are legal HTML. The best way to do that is by running your documents through one or more HTML validators.
A lengthier answer to this question is also available on this site if the explanation above did not satisfy you.
Who owns/maintain the Markup Validator?
The Markup Validator is maintained at W3C by W3C staff and benevolent collaborators, who receive a lot of help from contributors (read the full credits).
What other validators are there?
We're doing our best to provide clear and reliable results as well as a good interface with the Markup Validator, but for some reason you may want to check other validators. Here are a few choices:
- Page Valet, part of the Site Valet suite.
- WDG's HTML validator
Looking for validators at W3C, but not the Markup Validator? Check out the list of validators at W3C, including well-known CSS validator, link checker, etc.
How does The Validator work?
The Validator is based on James Clark's
nsgmls SGML parser. The Validator itself is a CGI script that fetches your URL, passes it through
nsgmls, and post-processes the resulting error list for easier reading.
How do I send feedback/bug reports about the Markup Validator?
Read the instructions on our Feedback page.
Using this service
How do I use this service?
Most probably, you will want to use the online Markup Validation service. The simple way to use this service to validate a Web page is to paste its address into the text area on the validator's home page, and press the "Check" button.
There are other possible uses and a few usage options, please read the user's manual for further help with this service.
If, for some reason, you prefer running your own instance of the Markup Validator, check out our developer's documentation.
What are these error messages?
The output of the Markup Validator may be hard to decipher for newcomers and experts alike, so we are maintaining a list of error messages and their interpretation, which should help.
Many error messages? Don't panic.
Don't panic. Did The Validator complain about your
DOCTYPE declaration (or lack thereof)? Make sure your document has a syntactically correct
DOCTYPE declaration, as described in the section on
DOCTYPE, and make sure it correctly identifies the type of HTML you're using. Then run it through The Validator again; if you're lucky, you should get a lot fewer errors.
If this doesn't help, then you may be experiencing a cascade failure — one error that gets The Validator so confused that it can't make sense of the rest of your page. Try correcting the first few errors and running your page through The Validator again.
Be patient, with a little time and experience you will learn to use the Markup Validator to clean up your HTML documents in no time.
I don't want error messages, I want you to clean up my page!
The Markup Validator can not do this for you. You may want to have a look at tools such as HTML Tidy.
My document is valid, can I use your "valid" icon?
Practical use of the icon
Yes. To show readers that one has taken some care to create an interoperable Web page, a "W3C valid" badge may be displayed (here, the "valid XHTML 1.0" badge) on any page that validates.
We encourage you to use the XHTML code below (or its HTML equivalent), but you may use a different code to integrate the icon within your web page as long as the icon is used as a link to revalidate the Web page it is in. Sample code is as follows:
alt="Valid XHTML 1.0!" height="31" width="88" /></a>
License and Guidelines for usage of the "valid" icons
Web content providers are granted the right to use the "W3C valid" logo on pages that pass validation (though the use of the W3C Markup Validator) for the W3C technology represented by the icon, and only on pages that pass validation. The icon must be used as a link to revalidate the Web page, thus providing a way to verify the page author's assertion that it passed validation.
Note that "W3C Valid" icons are not an endorsement by the W3C of the page's author, the substantive content of the page, nor its design. Instead, the icons are only a mechanism to identify pages that have been determined to be valid, and to easily re-validate pages as often as as they are modified.
Consequently, the use of the badge is in accordance with and governed by the W3C Trademark License and Logo and Icon usage policy.
Miscellaneous (Very) Frequently Asked Questions
No DOCTYPE Declaration Found!
A DOCTYPE Declaration is mandatory for most current markup languages and without one it is impossible to reliably validate a document.
One should place a DOCTYPE declaration as the very first thing in an HTML document. For example, for a typical XHTML 1.0 document:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
<html xmlns="http://www.w3.org/1999/xhtml" lang="en" xml:lang="en">
<!-- ... body of document ... -->
For XML documents, you may also wish to include an "XML Declaration" even before the DOCTYPE Declaration, but this is not well supported in older browsers. More information about this can be found in the XHTML 1.0 Recommendation.
The W3C QA Activity maintains a List of Valid Doctypes that you can choose from, and the WDG maintains a document on "Choosing a DOCTYPE".
No Character Encoding Found!
An HTML document should be served along with its character encoding.
Specifying a character encoding is normally done in the web server configuration file or administration program. The W3C I18N Activity has collected a few tips on how to do this in popular web server implementations.
IANA maintains the list of official names for character sets and the WDG has some information to help you correctly specify the character encoding.
To quickly check whether the document would validate after addressing the missing character encoding information, you can use the "Encoding" form control (accesskey "2") earlier in the page to force an encoding override to take effect. "iso-8859-1" (Western Europe and North America) and "utf-8" (Universal, but not commonly used in legacy documents) are common encodings if you are not sure what encoding to choose.
The validator complains about "&" in my URLs!
Most probably, you should read the ampersand section of WDG's excellent "common validation problem"
Most probably, you should read the script section of WDG's excellent "common validation problem"
HTML is based on SGML and uses an SGML feature (called SHORTTAG) (note that this is not the case with XHTML).
With this feature enabled, the "/" in <link ... /> or <meta ... /> already closes the link (or meta) tag, and the ">" becomes some regular text, which is not allowed in the <head> element. Since </head><body> is optional in HTML (again, not in XHTML), it is silently inserted, thus head-only elements like meta and style as well as "</head>" and "<body>", which may appear only once, become false.
(explanation courtesy of Christoph Päper)
I found some nasty typo like <p<a ...> and the validator accepted it!
This again (as in the previous case) comes from the SHORTTAG feature in HTML (not in XHTML). The typo is actually a "shorthand markup" and is a valid construct in HTML, even though its use is not recommended.
/check/referer does not work - or - the validator says it does not support my "undefined" URL scheme
Browsers and other Web agents usually send information about the page they come from, in a
Referer header. The validator uses this information for a features that allows it to validate whatever page the browser last visited. The "valid" icons on some Web page usually point to the validation of the page using this feature.
Unfortunately, some zealous "security software" or Web proxies strip the referrer information from what the browser sends. Without this information the validator is not able to find what the URL of the document to validate is, and gives the same error message as when it is given a type of URL it does not understand.
How to fix:
- Check that it is indeed the
Referer issue. The validator should have redirected you to
http://validator.w3.org/check?uri=. Otherwise, check the address you have given the validator.
- The validator can not fix this issue. You will have to (ask your administrator to) reconfigure whichever zealous software is stripping this referrer info.
- If you have a link on your page using the "/check/referer" feature, you could replace them with the a link to the validator without this feature, e.g.
- If you have no control over the page or annoying software, simply append the address of the page you wanted validated to the