Good news! So if you have apache2 already installed, you install W3C Validator and Perl:

sudo apt-get install w3c-markup-validator libapache2-mod-perl2

The issue is that the w3c-markup-validator package hasn't been updated to install properly on 13.10+ (I'm on 14.04). To fix it manually:

sudo ln -s /etc/w3c/httpd.conf /etc/apache2/conf-enabled/w3c-markup-validator.conf

I then had an issue where /usr/lib/cgi-bin didn't have the correct permissions. This is due to a problem in /etc/apache2/conf-available/serve-cgi-bin.conf where it will only give the correct permission if the module is being loaded. It appears this version of perl isn't listed in the IfModule statement. To fix this:

sudo gedit /etc/apache2/conf-available/serve-cgi-bin.conf

You want it to look like this:

<IfModule mod_alias.c>

    <IfModule mod_cgi.c>

        Define ENABLE_USR_LIB_CGI_BIN

    </IfModule>



    <IfModule mod_cgid.c>

        Define ENABLE_USR_LIB_CGI_BIN

    </IfModule>



    <IfModule mod_perl.c>

        Define ENABLE_USR_LIB_CGI_BIN

    </IfModule>



    <IfDefine ENABLE_USR_LIB_CGI_BIN>

        ScriptAlias /cgi-bin/ /usr/lib/cgi-bin/

        <Directory "/usr/lib/cgi-bin">

            AllowOverride None

            Options +ExecCGI -MultiViews +SymLinksIfOwnerMatch

            Require all granted

        </Directory>

    </IfDefine>

</IfModule>



# vim: syntax=apache ts=4 sw=4 sts=4 sr noet

Once you're done with these steps you have to symlink validator's html directory to /var/www/html:

sudo ln -s /usr/share/w3c-markup-validator/html /var/www/html/w3c-validator 

Once the file is linked and the changes are in place, all you have to do is restart the Apache server:

sudo service apache2 restart

Check that it's all working

http://localhost/w3c-validator

CSS might be missing from validator page in the browser, but validation should work well.