A few people asked me if Whidbey validation and intellisense schemas are compatible with VS 2003. Well, yes and no. You do can place Whidbey XHTML schemas (along with referenced W3C DOM type libraries for the client script intellisense) in VS 2003\Common7\Packages\Schemas\HTML and VS 2003 will recognize them. However, VS 2003 validation engine does not support many important XHTML validation features such as validation of missing or malformed closing tags, minOccurs/maxOccurs, validation of required attributes, uppercase/lowercase issues and few others. So caveat emptor and don't call tech support , please :-) Design view of VS 2003 is not XHTML-compliant, so don't use for XHTML it since it will turn your <br /> into <br>.
Whidbey should be XHTML-compliant and if it is not or we are missing something important, please let us know either here or in VS Whidbey Bug Reports Forum.