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The Nu Html Checker (v.Nu) is the backend of checker.html5.org, html5.validator.nu, and validator.w3.org/nu. Its source code is available, as are instructions on how to build, test, and run the code. It is released as two packages:

Note: The vnu.jar and vnu.war packages require a Java 8 environment; they won’t run in Java 7 or older environment.

You can get the latest release or run npm install vnu-jar, pip install html5validator or brew install vnu, and see the Usage and Web-based checking sections below. Or automate your HTML checking with a frontend such as:

Usage · Options · Web-based checking

Usage

Use the vnu.jar HTML checker as an executable for command-line checking of documents by invoking it like this: 

  java -jar ~/vnu.jar [--errors-only] [--Werrors] [--exit-zero-always]
       [--asciiquotes] [--no-stream] [--format gnu|xml|json|text]
       [--filterfile FILENAME] [--filterpattern PATTERN]
       [--html] [--skip-non-html] [--no-langdetect]
       [--help] [--verbose] [--version] FILES

Note: In these instructions, replace "~/vnu.jar" with the actual path to the file on your system.

To check one or more documents from the command line: 

  java -jar ~/vnu.jar FILE.html FILE2.html FILE3.HTML FILE4.html...

Note: If you get a StackOverflowError error when using the vnu.jar file, try adjusting the thread stack size by providing the -Xss option to java:

  java -Xss512k -jar ~/vnu.jar FILE.html...

To check all documents in a particular directory: 

  java -jar ~/vnu.jar some-directory-name/

To check all documents in a particular directory, skipping any documents whose names don’t end with the extensions .html, .htm, .xhtml, or .xht: 

  java -jar ~/vnu.jar --skip-non-html some-directory-name/

To check a Web document: 

  java -jar ~/vnu.jar URL

  example: java -jar ~/vnu.jar http://example.com/foo

To check standard input: 

  java -jar ~/vnu.jar -

  example: echo '<!doctype html><title>...' | java -jar ~/vnu.jar -

Options

When used from the command line as described in this section, the vnu.jar executable provides the following options:

--asciiquotes yes | no

Specifies whether ASCII quotation marks are substituted for Unicode
smart quotation marks in messages.

default: "no"

possible values: "yes" or "no"

--errors-only

Specifies that only error-level messages and non-document-error messages
are reported (so that warnings and info messages are not reported).

default: [unset; all message reported, including warnings & info messages]

--Werrors

Makes the checker exit non-zero if any warnings are encountered (even if
there are no errors).

default: [unset; checker exits zero if only warnings are encountered]

--exit-zero-always

Makes the checker exit zero even if errors are reported for any documents.

default: [unset; checker exits 1 if errors are reported for any documents]

--filterfile FILENAME

Specifies a filename. Each line of the file contains either a regular
expression or starts with "#" to indicate the line is a comment. Any error
message or warning message that matches a regular expression in the file is
filtered out (dropped/suppressed).

default: [unset; checker does no message filtering]

--filterpattern PATTERN

Specifies a regular-expression pattern. Any error message or warning
message that matches the pattern is filtered out (dropped/suppressed).

default: [unset; checker does no message filtering]

--format format

Specifies the output format for reporting the results.

default: "gnu"

possible values: "gnu", "xml", "json", "text" [see information at URL below]

https://github.com/validator/validator/wiki/Service-%C2%BB-Common-params#out

--help

Shows detailed usage information.

--skip-non-html

Skip documents that don’t have *.html, *.htm, *.xhtml, or *.xht extensions.

default: [unset; all documents found are checked, regardless of extension]

--html

Forces any *.xhtml or *.xht documents to be parsed using the HTML parser.

default: [unset; XML parser is used for *.xhtml and *.xht documents]

--no-langdetect

Disables language detection, so that documents are not checked for missing
or mislabeled html[lang] attributes.

default: [unset; language detection & html[lang] checking are performed]

--no-stream

Forces all documents to be be parsed in buffered mode instead of streaming
mode (causes some parse errors to be treated as non-fatal document errors
instead of as fatal document errors).

default: [unset; non-streamable parse errors cause fatal document errors]

--verbose

Specifies "verbose" output. (Currently this just means that the names of
files being checked are written to stdout.)

default: [unset; output is not verbose]

--version

Shows the vnu.jar version number.

Web-based checking with vnu.war or vnu.jar

The Nu Html Checkerーalong with being usable as a standalone command-line clientーcan be run as an HTTP service, similar to checker.html5.org, html5.validator.nu, and validator.w3.org/nu, for browser-based checking of HTML documents over the Web. To that end, the checker is released as two separate packages:

Both deployments expose a REST API that enables checking of HTML documents from other clients, not just web browsers. And the vnu.jar package also includes a simple HTTP client that enables you to either send documents to a locally-running instance of the checker HTTP serviceーfor fast command-line checkingーor to any remote instance of the checker HTTP service running anywhere on the Web.

The latest releases of the vnu.jar and vnu.war packages are available from the validator project at github. The following are detailed instructions on using them.

Note: Replace "~/vnu.jar" or "~/vnu.war" below with the actual paths to those files on your system.

Standalone · Servlet · Client · Options

Standalone web server

To run the checker as a standalone service (using a built-in Jetty server), open a new terminal window and invoke vnu.jar like this: 

  java -cp ~/vnu.jar nu.validator.servlet.Main 8888

Then open http://localhost:8888 in a browser. (To have the checker listen on a different port, replace 8888 with the port number.)

You’ll see a form similar to validator.w3.org/nu that allows you to enter the URL of an HTML document and have the results for that document displayed in the browser.

Note: If you get a StackOverflowError error when using the vnu.jar file, try adjusting the thread stack size by providing the -Xss option to java: 

  java -Xss512k -cp ~/vnu.jar nu.validator.servlet.Main 8888

Deployment to servlet container

To run the checker inside of an existing servlet container such as Apache Tomcat you will need to deploy the vnu.war file to that server following its documentation. For example, on Apache Tomcat you could do this using the Manager application or simply by copying the file to the webapps directory (since that is the default appBase setting). Typically you would see a message similar to the following in the catalina.out log file. 

May 7, 2014 4:42:04 PM org.apache.catalina.startup.HostConfig deployWAR
INFO: Deploying web application archive /var/lib/tomcat7/webapps/vnu.war

Assuming your servlet container is configured to receive HTTP requests sent to localhost on port 80 and the context root of this application is vnu (often the default behavior is to use the WAR file's filename as the context root unless one is explicitly specified) you should be able to access the application by connecting to http://localhost/vnu/.

Note: You may want to customize the /WEB-INF/web.xml file inside the WAR file (you can use any ZIP-handling program) to modify the servlet filter configuration. For example, if you wanted to disable gzip decompression you could comment out that filter like this: 

<!--
  <filter>
      <filter-name>gzip-filter</filter-name>
      <filter-class>org.mortbay.servlet.GzipFilter</filter-class>
  </filter>
  <filter-mapping>
      <filter-name>gzip-filter</filter-name>
      <url-pattern>*</url-pattern>
  </filter-mapping>
-->

HTTP client (for fast command-line checking)

You can also use vnu.jar from the command line to either send documents to a locally-running instance of the checker HTTP serviceーfor fast command-line checkingーor to a remote instance anywhere on the Web.

To check documents locally, do this:

  1. Start up the checker as a local HTTP service, as described in the Standalone web server section.
  2. Open a new terminal window and invoke vnu.jar like this: 
      java -cp ~/vnu.jar nu.validator.client.HttpClient FILE.html...
To send documents to an instance of the checker on the Web, such as html5.validator.nu/, use the nu.validator.client.host and nu.validator.client.port options, like this: 
  java -cp ~/vnu.jar -Dnu.validator.client.port=80 \
     -Dnu.validator.client.host=html5.validator.nu \
     nu.validator.client.HttpClient FILE.html...

Other options are documented below.

HTTP client options

When using vnu.jar for sending documents to an instance of the checker HTTP service for checking, you can set Java system properties to control configuration options for the checker behavior.

For example, you can suppress warning-level messages and only show error-level ones by setting the value of the nu.validator.client.level system property to error, like this: 

   java -Dnu.validator.client.level=error\
       -cp ~/vnu.jar nu.validator.client.HttpClient FILE.html...

Most of the properties listed below map to the validator.nu common input parameters documented at github.com/validator/validator/wiki/Service:-Common-parameters.

nu.validator.client.host

Specifies the hostname of the checker for the client to connect to.

default: "127.0.0.1"

nu.validator.client.port

Specifies the hostname of the checker for the client to connect to.

default: "8888"

example: java -Dnu.validator.client.port=8080 -jar ~/vnu.jar FILE.html

nu.validator.client.level

Specifies the severity level of messages to report; to
suppress warning-level messages, and only show error-level ones, set
this property to "error".

default: [unset]

possible values: "error"

example: java -Dnu.validator.client.level=error -jar ~/vnu.jar FILE.html

nu.validator.client.parser

Specifies which parser to use.

default: "html"; or, for *.xhtml input files, "xml"

possible values: [see information at URL below]

https://github.com/validator/validator/wiki/Service-%C2%BB-Common-params#parser

nu.validator.client.charset

Specifies the encoding of the input document.

default: [unset]

nu.validator.client.content-type

Specifies the content-type of the input document.

default: "text/html"; or, for *.xhtml files, "application/xhtml+xml"

nu.validator.client.out

Specifies the output format for messages.

default: "gnu"

possible values: [see information at URL below]

https://github.com/validator/validator/wiki/Service-%C2%BB-Common-params#out

nu.validator.client.asciiquotes

Specifies whether ASCII quotation marks are substituted for Unicode
smart quotation marks in messages.

default: "yes"

possible values: "yes" or "no"

HTTP servlet options

nu.validator.servlet.connection-timeout

Specifies the connection timeout.

default: 5000

possible values: number of milliseconds

example: -Dnu.validator.servlet.connection-timeout=5000

nu.validator.servlet.socket-timeout

Specifies the socket timeout.

default: 5000

possible values: number of milliseconds

example: -Dnu.validator.servlet.socket-timeout=5000

Build instructions

Follow the steps below to build, test, and run the checker such that you can open http://localhost:8888/ in a Web browser to use the checker Web UI.

  1. Make sure you have git, python, and JDK 8 installed.
  2. Set the JAVA_HOME environment variable: 
    export JAVA_HOME=/usr/lib/jvm/java-8-openjdk-amd64    <-- Ubuntu, etc.
    export JAVA_HOME=$(/usr/libexec/java_home)            <-- MacOS
  3. Create a working directory: 
    git clone https://github.com/validator/validator.git
  4. Change into your working directory: 
    cd validator
  5. Start the build script: 
    python ./build/build.py all

    The first time you run the build script, you’ll need to be online and the build will need time to download several megabytes of dependencies.

The steps above will build, test, and run the checker such that you can open http://localhost:8888/ in a Web browser to use the checker Web UI.

Use python ./build/build.py --help to see command-line options for controlling the behavior of the script, as well as build-target names you can call separately; e.g.: