Thanks for your message. You don't really have to worry about the validation as ASP.NET itself cannot produce completely valid code. The validator requires a lot of things that ASP.NET itself breaks.
However, this does not mean dysfunctional pages or display error at all, as you can check with all major browsers. What is more important than that validator is that we support all major browsers and everything displays correctly under the XHTML 1.0 Strict Doctype. We are not coding the software in quirks mode to ensure best cross platform compatibility and HTML rendering efficiency.
In addition, we had to introduce some CSS workarounds too for IE 7 to make it display pages like the rest of the more compliant browsers. These workarounds are by themselves not "compliant", but actually make the software more "compatible" with more browsers.