Thanks for your replies.
I know accessibility and compliant HTML are different issues but I have read interviews from Tim O' Brien on Lee Syke's website where they claim that if the document definition is changed to XHTML and some changes are made in default.aspx with minor changes in ascx controls for modules, the website completely validates to W3C standards.
Ofcourse he has used just text/html module on his site. Those changes would need to be spread over all the other ascx controls for the modules. But still, that would be the first step for an accessible website.......completely valid HTML.
Is there any reason why is this process not implemented in the default installation of dotnetnuke??