Needs: Apache, Perl
The W3C (X)HTML validator is now available as package for the majority
of Linux distributions. If no such package is available for your OS,
you can always fall back on the W3C distribution. The installation
instructions are pretty staight forward and easy to follow.
For Mac OS X (10.4) I had to copy the conf/*.conf files to /etc/w3c and adjust the Apache config file a bit, to reflect my installation.
Needs: java jdk/jre, Jigsaw or Tomcat
The W3C CSS validator can only be checked-out from the CVS repository. It looks, at the time of this writing (May 2007), that this checkout is not quite right as some removed files still show up and will make installation not possible for javac version 1.5.0_08 or above.
To run the CSS validator, you'll need an applet server. After having a struggle with Tomcat on Ubuntu, I chose Jigsaw and it is by far the easiest way to start. Installation is straight forward