When using the Telerik AJAX Panel, on postback you may find that the updated controls will use the wrong stylesheet. Here is an article detailing the issue:
The article mentions no workaround, however, since you are working in a .NET environment you do have the option of server side browser detection and then push the proper CSS for the browser. This would bypass the bug in the conditional comments.
Another option which is recommended in the link above is using HeadJS which will detect browser and apply the correct styles.