ASP.NET valid XHTML

Hello,

I am trying to create simple web form in Visual Studio. But any form is not valid HTML. Tidy validator (http://tidy.sourceforge.net) says there is a warning

Warning: <input> ID "__VIEWSTATE" uses XML ID syntax

Does anybody know solution? How to create Warning free XHTML form in .NET?

btw http://www.kirupafx.com/HelloWorld/Default.aspx has the same warning.

Thanks