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Installing The W3C HTML Validator With HTML5 Support (On Ubuntu)

We use the W3C HTML validator for driving HTML validation tests.

We don’t use the W3C public service at; the usage limits imposed (quite fairly) by the W3C are too restrictive for our needs, and the latency involved in sending HTML out to the Internet to be validated increases by orders of magnitude the time it takes to complete an HTML validation test.

We install a local copy of the W3C HTML validator and a local copy of the HTML5 validato.

If you want to frequently validate many HTML documents quickly, here’s how you can do the same.

Installing the W3C HTML Validator

Installing from the Debian package is a breeze: sudo apt-get install w3c-markup-validator

That’ll get you the basic installation. The W3C HTML validator will now work for any local virtual host via http://{hostname}/w3c-validator/.

Configuring the W3C HTML Validator

The W3C HTML validator, by default, only accepts URLs of pages that exist on the public Internet. I have no idea why; if you’re installing the validator locally, might you be wanting to validate locally-hosted HTML?


Sorry, the IP address of is not public. For security reasons, validating resources located at non-public IP addresses has been disabled in this service.

You need to tell the validator to allow local IPs. Edit /etc/w3c/validator.conf and change Allow Private IPs = no to Allow Private IPs = yes.

Installing the HTML5 Validator

The HTML5 spec is still under development and as such support is not provided out the box. Let’s try anyway:


Checking the Document Type of this document requires the help of an external tool which was either not enabled in this validator, or is currently unavailable. Check in the validator’s system configuration that HTML5 Validator is enabled and functional.

Looks like we need to install the HTML5 validator.

The HTML5 validator uses a bit of an ad-hoc build system dependent on Python, Mercurial and Subversion. Make sure these are installed:

sudo apt-get install mercurial subversion python

The validator is Java-based, so check first that that you have a JDK available and check the location. It’s probably to be found at /usr/lib/jvm/java-6-openjdk. Once you’re ready, follow the

You’ll need to make a directory into which to install the HTML5 validator. Anywhere will do, I opted for /usr/share/html5-validator.

Running the following as root worked for me:

mkdir /usr/share/html5-validator && cd /usr/share/html5-validator
export JAVA_HOME=/usr/lib/jvm/java-6-openjdk
hg clone build
python build/ all
python build/ all

You’ll end up with the HTML5 validator running in a standalone web server on port 8888:

2012-08-13 14:28:19.514:INFO::jetty-6.1.26
2012-08-13 14:28:19.549:INFO::Started SocketConnector@

Running the HTML5 Validator as a Service

I want the HTML5 validator available all the time without having to keep a terminal window open and without needing to manually start up the validator every time a worker server starts up.

To do this, you need an init script to start up a daemon. I created /etc/init/html5-validator.conf containing:

description " HTML5 Validator"

start on runlevel [234]
stop on runlevel [0156]

chdir /usr/share/html5-validator
exec python build/ --control-port=8889 run

I can now start/stop the HTML5 validator using: sudo service html5-validator start/stop

Configuring the W3C HTML Validator for HTML5

One last step: we need to enable HTML5 validation for the W3C validator.

Simply edit /etc/w3c/validator.conf and uncomment the line HTML5 = http://localhost:8888/html5/HTML. That’s it.

Let’s give things a go again:


Line 619, Column 172: An img element must have an alt attribute, except under certain conditions. For details, consult guidance on providing text alternatives for images.

Excellent. The W3C HTML validator is running locally and is handling HTML5 documents. Now I just need to make sure the markup I’m making is valid.