The W3C Markup Validator provides Perl/CGI/SGML/XML/DTD-based validation of a variety of document types.
SGML and DTDs are older technologies that never found wide use on the Web, so for checking of HTML documents using modern technologies, you probably want to instead use the W3C HTML Checker.
To do that, Rather than trying to install and run an instance of the W3C from the sources, it's much easier to install one of a variety of pre-built packages.
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Fedora RPM packages of the validator are included in Fedora.
The name of the validator package is A Debian package is available, courtesy of Frédéric Schütz.
Starting with Debian 3.1 ("Sarge"), the package and all its dependencies are included in the official Debian distribution, and can be installed by running the command Software License.If you just want to glance at the code, or see its revision history, you can browse it directly in Github.The most interesting files are currently a CGI script called "check" that does pretty much everything, and possibly also the configuration file snippet for Apache.Select the topmost revision numbers on these pages to see the most recent revision of each file.To actually install and run an instance of the W3C Markup Validator from the sources, see the installation manual.The Internet has been praised as a medium that truly frees information, making it accessible to anyone, anywhere.