Our controls are xhtml compliant and they work and look as expected when you have the corresponding <!DOCTYPE> declaration.
Other controls, however, are designed to work correctly in quirksmode and that is why a conflict may occur.
We can not tell what exactly happens in your project but it is most probably related to the page's mode. Since we do not have your code we are not able to help because these problems are very specific.
Would you please send us a sample page with a detailed explanation of the problem along with some screenshots? Once we receive it we will do our best to help.
the Telerik team