Since all the documents in the world are getting converted to HTML format (or being in the process), the HTML beautifier is immensely important. Each and every document, book, articles, news and papers about science, technology, medicine, politics and others are already available or in the process of getting converted to HTML documents. So HTML deservers a separate chapter like this one.
HTML : Clean up HTML and fix mistakes with Tidy http://tidy.sourceforge.net See also validate HTML files
Apache Module HTML validator: Apache: mod_tidy
Perl HTML::Validator at HTMl::Validator . Download this module and build the 'validate' command with 'make install'.
The WebLint PERL HTML validator at HTML::Lint and at WebLint HTML Validator
CSE HTML Validator at HTML Validator
HTML : "http://www.datacomm.ch/mwoog/software/perl/beautifier.html"
HTML : "http://www.firstpage.com"
HTML : "http://www.trita.com"