Producing XHTML-Compliant Pages With Response Filters

Programming with web standards in mind, although vastly ignored, is becoming more and more important. It almost seems it took too long to promote ASP.NET. Now that we're over the hill and "this stuff works" it is about time to start paying attention to web standards. In this article you will learn how to implement a response filter and plug it into the ASP.NET pipeline. The filter will transform outgoing HTML into XHTML 1.0-compliant markup.

original article from http://www.aspnetresources.com/articles/HttpFilters.aspx