About the Markup Validator
Help me! I clicked on an icon and ended up on this strange site!
Don't panic!
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. The (X)HTML languages, for all versions up
to XHTML 1.1, are using machine-readable grammars called
DTD
s, 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.
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".
No, they are different concepts.
Markup languages are defined in
technical specifications
,
which generally include a
formal grammar
.
A document is valid when it is correctly written in accordance
to the formal grammar, whereas conformance relates to the
specification itself. The two
might
be equivalent, but in most cases,
some conformance requirements cannot be expressed in the grammar, making validity
only a part of the conformance.
What is the Markup Validator and what does it do?
The Markup Validator is a free tool and service that
validates markup
:
in other words, it checks the syntax of Web documents, written in formats
such as (X)HTML.
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 and the languages it can validate.
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?
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 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!
Have a look at tools such as
HTML Tidy
and
tidyp
. When selected, the
"Clean up Markup with HTML-Tidy" option will output a "cleaned"
version of the input document in case it was not valid, done with
HTML-Tidy
, using
the Markup Validator's default HTML-Tidy configuration. Note that there
are no guarantees about the validity or other aspects of that output,
and there are many options to configure in these tools that may result
in better clean up than the Validator's default options for your
document, so you may want to try out them locally.
Miscellaneous (Very) Frequently Asked Questions
No DOCTYPE Declaration Found!
A DOCTYPE Declaration is mandatory for HTML documents.
Unless you have very specific needs, you should use the following generic DOCTYPE:
<!DOCTYPE html>
. A typical HTML document looks like:
<!DOCTYPE html>
<html lang="en">
<head>
<title>Title</title>
</head>
<body>
<!-- ... body of document ... -->
</body>
</html>
No Character Encoding Found!
An HTML document should be served along with its character encoding.
Specifying a character encoding is typically done by the web server
configuration, by the scripts that put together pages, and inside the
document itself.
IANA
maintains the list of
official names for character
encodings
(called charsets in this context). You can choose from a number
of encodings, though we recommend UTF-8 as particularly useful.
The W3C
I18N
Activity has collected a
few tips on
how to do this
.
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, and more commonly used in recent documents) are
common encodings if you are not sure what encoding to choose.
/check?uri=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.
Also, requests to non-secure HTTP resources from links in documents
transferred with a secure protocol such as HTTPS should not include
referrer information
per the HTTP/1.1 specification
.
As the validator at validator.w3.org is currently not available over
HTTPS, this referrer feature will not work reliably for documents
transferred over secure protocols (usually
https
URLs)
with it.
How to fix
:
- Check that it is indeed the
Referer
issue. The validator should have redirected you to
https://validator.w3.org/check?uri=
your_url_here
. Otherwise, check the address you have given the validator.
- The validator cannot 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?uri=referer" feature, you could replace them with the
a link to the validator without this feature, e.g.
https://validator.w3.org/check?uri=http%3A%2F%2Fwww.example.com
- If you have no control over the page or annoying software, or your page's URL is a
https
one, simply append the address of the page you wanted validated (URI encoded)
to the
https://validator.w3.org/check?uri=
address.