Freezing header in asp.net gridview with DOCTYPE declaration

I know that this topic has already discussed a lot. But, what I face today is some more strange;

My environment is, Win 7, Visual Studio 2008, ie8

I created a sample aspx and added a gridview, could make it work nicely using below style;

thead tr
        {
            position: relative;
            top: expression(this.offsetParent.scrollTop);
        }

only when I comment out automatically generated D开发者_开发百科OCTYPE declaration;

How can I make this webform work correctly with DOCTYPE declaration is on?

You can't because the doctype won't let you use the 'expression' because that is crazy talk.