Which Target Schema For Validation Should I Set For Web Form ?

Hello,

 Can anyone tell me which "Target Schema for Validation" should I set for a web form?  I noticed that if I use XHTML 1.1 for Target Schema for Validation and switch to different target schema like HTML 4.01 and gets a couple of attribute element errors. I just want to set the right target scheme to fit in different web browser like a  FireFox (Mozilla),  Internet Explorer 7 or NetScape 7 and I'm using VS 2005 for my web application?  thanks
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