As far as I can see, the current official version of RadDatePicker produces valid XHTML. Please provide some more information :
1) Which HTML validator do you use?
2) What is the error?
3) What is the control declaration, outputting invalid XHTML?
By the way, the <link> tags are added to the page by a mechanism, which is used by all RadControls, so if a problem exists, it should be observed with all controls.
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