Make sure that you are using XHTML-compliant Doctype in the content and the parent page. e.g.:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
If this doesn't help, please open a support ticket and send us a small sample project where the problem could be reproduced so we can investigate further.
All the best,
the Telerik team
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