The HTML 2.0 Standard defines 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.
The service currently offers validation testing with respect to a draft of the the HTML 3.0 DTD and a few other DTDs in development.
Gerald Oskoboiny offers a "Kindler, Gentler" version of the validation service. It has much nicer diagnostic messages.
We're also investigating ways to test browsers, including a certification process.
For an idea of how this might work, please see: