The W3C HTML 3.2 and HTML 4.0 Recommendations define conformance of HTML documents (web pages) and certain aspects of HTML user agents (web browsers).
In order to promote reliability and fidelity of communications on the web, W3C is coordinating testing efforts. See: the W3C Validation Service.
The HTML Validation Service and Toolkit by webtechs allows you to check that your documents conform to the HTML syntax. (There are certain application conventions that are not checked by those tools.)
March 1999: webtechs.com falls victim to Network Solutions
The service currently offers validation testing with respect to HTML 2.0, HTML 3.2, a draft of the HTML 3.0 DTD, a "mozilla" DTD, the SoftQuad HotMetaL PRO 2.0HTML DTD and the Microsoft Internet Explorer 2.0 DTD.
Weblint checks style as well as syntax.
The Spyglass HTML Validator is a Windows95 application that can be downloaded and used to check against four DTDs:
We're also investigating ways to test browsers, including a certification process.
For an idea of how this might work, please see: