Tools - Refactoring HTML: Improving the Design of Existing Web Applications - page 30

It is usually not necessary to check each and every page on your site for these sorts of errors. Most errors
repeat themselves. Generally, once you identify a class of errors, it becomes possible to find and automatically
fix the problems. For example, here I could simply search for <blockquote source= to find other places where I
had made the same mistake.
This page was actually cleaner than normal to begin with. I next tried one of the oldest and least-maintained
pages I could find on my sites. This generated 144 different errors.
Private Validation
The W3C is reasonably trustworthy. Still, you may not want to submit private pages from your intranet
to the site. Indeed, for people dealing with medical, financial, and other personal data, it may be illegal
to do so. You can download a copy of the HTML validator scripts from http://validator.w3.org/docs/install
and install them locally. Then your pages don't need to travel outside your firewall when validating.