Valid HTML
Valid HTML refers to an HTML document that has been passed through an HTML validation service, or syntax checker, and has proven to be syntactically and formally correct. While there are numerous HTML checkers and validation services available on the Internet, the best-known service is run by Mark Gaither at WebTechs, called the HTML Validation Service. Files that have been validated show an icon at the bottom of each page.
The HTML Validation Service allows a user to input an HTTP-based URL in a form, choose one of three DTDs (Level 2 for HTML 2.0, Level 3 for the now-defunct HTML 3.0, and Mozilla for the Netscape DTD), choose either strict or standard checking, and then submit the form to the service. The service is built upon James Clark's sgmls version 1.1.19 SGML parser and uses a simple WWW client to retrieve a URL. Users can also enter bits and pieces of HTML in another part of the form.
Validation queries result in either a pass or a fail. Upon failure, users will see a list of cryptic sgmls error messages that can be helpful in correcting nonstandard uses of HTML in a document. Upon passing, the server presents the user with a congratulatory page and invites them to place a "valid HTML" icon on their page(s). The icon indicates which DTD was applied, as shown in the valid HTML icon at the foot of this page.
WebTechs also offers the HTML Check Toolkit, which allows users to instruct WebTechs' server to custom build a validation toolkit for their local use. The user inputs into an HTML form that requires the user to determine such things as the native UNIX operating system in use at their site (yes, this means UNIX is required), the directory paths where binary files and man pages should be installed, and what kind of compression algorithm to use.
The biggest benefit of the HTML Check Toolkit is that it lets organizations validate their HTML documents locally, without having to send them elsewhere across the Web. This approach is strongly recommended for sites that produce HTML documents in any kind of quantity, and for those that mandate valid HTML. It's even possible to build specialized forms and CGI applications around this toolkit (which is how the WebTechs Validation Service was built). The HTML service and the toolkit are both free, and are located at http://www.webtechs.com/html-tk/.