I do not thing that there are css classes that we load and which could break your controls. The styles that applies over a control comes from your template or these are the styles that are loaded as embedded resources from the control. If you use RadControls for ASP.NET AJAX you can set EnableEmbeddedScripts, EnableEmbeddedSkin and EnableEmbeddedBaseStylesheet when you are in design or preview mode depending on your needs, but as I said you should relevant resources that would ensure that all functions of the control will keep working properly.
WebUserControl1 : System.Web.UI.UserControl
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